Upgraded my video card to the above mentioned 3DImage 9750 and now zgv displays only color garbage. xzgv works fine. I have also dl'd the source and re-compiled (w/o errors) with the same results. The maintainer apparently is not directly available as all his email addresses in the source code and at sourceforge bounce.
Finally got a hold of maintainer Russell Marks and he said that it was an svgalib problem. Contacted svgalib maintainer and he suggested adding: chipset VESA to /etc/vga/libvga.config. This worked. This also works with zgv 5.1 and this particular video card.
many thanks for your solution. I thinks, i will make VESA in /etc/vga/libvga.config as default. Now many people have the new graphic card. which already support VESA.