256mb vs 512 mb on a KT7

[256]

I have 512mb (2x256 crucial Pc133 cas2 -7E) and when both chips are used in the system, my 3dmarks and FPS in most games drops by half. removing one or the other memory module causes the system to resume its regular speed. i'm using an athlon 1g at 1.25 and an inno3d GF2 PRo64 w. 5ns. I have swapped both modules out to a different Kt7-a sys and the same bloody thing occurs. have tested it on win2k, ME and 98 with the same results.. any ideas?