It appears that you have installed the M5953LL/A VRAM upgrade kit which is incorrect for this video card. This kit contains two 256K SIMMs and should be used in the Power Macintosh 7100/66 and other computers which accept 256K VRAM SIMMs.
The correct VRAM upgrade kit is M1311LL/A, which contains one 512K VRAM SIMM. The Power Macintosh 8100 video card requires four 512K VRAM SIMMs to upgrade from 2MB to 4MB VRAM.