Framerate worse in sli!!?

I recently bought a new computer with dual 8800gtx cards in sli and i am finding that im getting a significant increase in framerate when i use only 1 card and disable sli and im using the latest drivers from nvidia. i have tried different types of settings for anti-aliasing and anistropic filtering. i have tried this in unreal tournament 3, crysis, counter strike source and age of conan. the only game that benefits from using sli is crysis. i have download vista service pack 1 and a few hotfixes but nothing seems to have worked. does anyone know why my computer seems to perform better with sli disabled? any help would be appreciated, my system specs are below.

dell XPS 720
2x 8800GTX
vista home premium sp1 32-bit
intel core 2 extreme quad 3ghz
4gb ram

not sure but its up to date and im getting a 25-30 fps increase when i use 1 card over the both of them