Note: the kernel version listed is FC11 - which works, sort of. I cannot remember which version of FC12, but it was the latest available from yum from 5.Jan to 8.Jan. I think it was 2.6.31.9-174 i686 PAE with akmod-nvidia. I spent about 30 hours with FC12 before giving up.
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I don't know if it is important, but I am using a dual core 2.7gHz AMD X64 processor, 4gB of DRAM, & no swap space.
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I replaced FC12 with FC11 as FC12 is not usable on this machine. I do use FC12 on my ASUS eeePC-1000, IBM T40 thinkpad, and a house server.
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As noted elsewhere S3 (suspend) is less than graceful on FC11. I believe I read that this would not be fixed in FC11. I am contemplating upgrading to FC10.
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This is my software development machine (eclipse: C, Python, TCL, Lua, SVN, and Google-code) and if I could afford the iron, I would have more large screens - each with 4 or more workspaces.

Bruce if you have hybrid sli enabled in bios then the hardware has to work like this... It wants to give one of the cards the frame buffer and let the other card fetch back and forth from the frame buffer. This means one card locks the frame buffer and hence the screen.
So we have 2 problems here. X and nouveau don't work with the 8400GS either the G86 or the G98 version. You have them working in SLI mode which disabled their ability to work as dual independant gpu's. Hence you are confusing any attempt X has at trying to arbitrate them into seperate gpu's.
Until g86 and g98 gpu is working you're stuck from trying nouveau.
Since you are using the nvidia driver though you should be able to get this to work. You do have to set SLI to disable though.

Roger that. Will test as FC12 doesn't even boot beyond "Starting udev". I now have 2 OS multiboot in place and soon as I can reorganize my HDD, it will be 3 OS's.
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my video config is best described as dual - single head (both DVI - no VGA {too fuzzy})
lspci bus info:
00: nVidia Corporation MCP78S [GeForce 8200] (rev a2)
01: Multimedia audio controller: Creative Labs SB Audigy (rev 04) 
02: VGA compatible controller: nVidia Corporation GeForce 8200 (rev a2) 
03: VGA compatible controller: nVidia Corporation GeForce 8400 GS (rev a1)

Ugh :-(. This is what BIOS looks like:
Advanced Settings | Chipset settings | South Bridge
- video priority: PCI card first, Internal VGA first, PCIe card first.
- Hybrid SLI mode: mGPU auto, mGPU enable
- Hybrid SLI frame buffer size: 64 ... 512 MB
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My original settings (works with FC11 kmod-nvidia): Internal VGA first, mGPU enabled, FB size= 256MB.
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Revised settings: PCIe card first, mGPU auto. FB size is greyed out. FC11 boots. BIOS messages go to the other screen, but X11 hangs. Booted to run level 3. lspci has the 8400 GS on bus 02: and no 8200 defined. Sort of defeats the purpose of two monitors, don't it?
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Revised settings: Internal VGA first. FB size no longer greyed out. Set to 256 MB (defaults to 512 MB, and X11 hangs at less than 256MB). Left mode at mGPU auto. FC11 boots normally with the above PCIe bus values. FYI GRUB KERNEL line has "vmalloc=256M" without which I get "no screen found for ...."
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Conclusion: No dual single-head support without SLI aka mGPU, at least for FC11 - my production guest OS. I can test the lspci assignments of FC12 with current config. Will report on lspci assignments.
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Background: I do software development and typically have 30 or so windows open. The development frameworks and test stuff are on the left screen and the documentation (html, pdf, and odt) are on the right screen. Each screen is 1920 x 1200, 24" LCD DVI with 4 Gnome workspaces. Typically 7 Gnome workspaces are in use. VGA connection is too fuzzy and dual DVI cards were too expensive. I see that there are some below $75 now.
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I have 4 LCD monitors on my desk (3 physical hosts) and there isn't any room for more. Boo hoo :-)

Same PCIe bus assignment behaviour in FC12 as in FC11. Clearly, I can never migrate to FC12. But, with FC13 coming out, hope lives on.
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I have other FC12 video problems with kmod-nvidia and the downloaded drivers from nVidia (v 195). The FC12 kernel will not boot beyond "Starting udev". See bug #594542. I have become much too practiced with installing FC-12 from scratch :-).

One last note: when I switch BIOS video settings I cannot restart. The physical host hangs in BIOS. A full power down, wait, boot from front panel button fixes the problem. I not not have to turn the power supply off, as I have to do with some FC12 video (xorg.conf) configurations.

FC-13 works! Yea!!!
kernel 2.6.33.5-112.fc13.i686.PAE
More importantly, it works with the stock nouveau drivers and does not require the vmalloc=256M
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The only problem was the lack of documentation on how to create a harmless xorg.conf that functioned exactly like the installation default (no xorg.conf). A sort of baseline to modify from.
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I have attached the xorg.conf that worked.
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# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 1.0  (buildmeister@builder63)  Tue May 12 12:47:54 PDT 2009

# Xorg configuration created by livna-config-display

Section "ServerLayout"
    Identifier     "single head configuration"
    Screen      0  "Screen0" 0 0
    Screen      1  "Screen1" RightOf "Screen0"
    InputDevice    "Mouse0" "CorePointer"
    InputDevice    "Keyboard0" "CoreKeyboard"
EndSection

# Section "Files"
#     ModulePath      "/usr/lib/xorg/modules/extensions/nvidia"
#     ModulePath      "/usr/lib/xorg/modules"
# EndSection

Section "ServerFlags"
    Option         "AIGLX" "on"
    Option         "Xinerama" "0"
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "auto"
    Option         "Device" "/dev/input/mice"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"

# keyboard added by rhpxl
    Identifier     "Keyboard0"
    Driver         "kbd"
    Option         "XkbModel" "pc105"
    Option         "XkbLayout" "us"
EndSection

Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "XXX M24EI5"
    HorizSync       30.0 - 83.0
    VertRefresh     56.0 - 61.0
    Option         "DPMS"
EndSection

Section "Monitor"
    Identifier     "Monitor1"
    VendorName     "Unknown"
    ModelName      "XXX M24EI5"
    HorizSync       30.0 - 83.0
    VertRefresh     56.0 - 61.0
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nouveau"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 8200"
    Option         "AddARGBGLXVisuals" "True"
    BusID          "PCI:2:0:0"
EndSection

Section "Device"
    Identifier     "Device1"
    Driver         "nouveau"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 8400 GS"
    Option         "AddARGBGLXVisuals" "True"
    BusID          "PCI:3:0:0"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "TwinView" "0"
#    Option         "metamodes" "nvidia-auto-select +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Section "Screen"
    Identifier     "Screen1"
    Device         "Device1"
    Monitor        "Monitor1"
    DefaultDepth    24
    Option         "TwinView" "0"
#    Option         "metamodes" "nvidia-auto-select +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Section "Extensions"
    Option         "Composite" "Enable"
EndSection

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