I have a Matrox Millennium I in a DIGITAL PersonalWorkstation 433a. The installer tries to run X in what appears to be 640x480. The display has alternating verticle bars of the correct graphic and a light blue (I assume it's the root window color). The Xconfigurator run by the text installer cannot probe the card. It incorrectly identifies the card as having 4MB of VRAM. Xconfigurator run from the installed system can correctly probe and generate a config file that works at 1024x768 (the default) and I also tried it at 640x480. It correctly identifies the card has having 2MB of VRAM.
The problem is that kudzu's cards database (in instimage/usr/share/kudzu/pcitable contains this entry: 0x102b 0x0519 "Card:Matrox Millennium 4MB" "Matrox|MGA 2064W [Millennium]" change it to this: 0x102b 0x0519 "Card:Matrox Millennium 2MB" "Matrox|MGA 2064W [Millennium]" and things work. Of course, this is just a hack, as I don't know for certain that there are no 1MB Matrox Millennium cards out there.
Probing memory on Matrox cards is not reliable yet - please manually choose the amount of RAM your card has.