2GB of RAM on 64-bit Home Premium. I'll probably be going higher soon, but that's been good enough for me so far.
My brother has a single nVidia card (8500) with 32-bit Home Premium and keeps telling me how bad nVidia drivers are compared to my ATI drivers. We even compared with a single HD2600 card, which should be a roughly comparable card (not quite, but good enough for generic comparison). I think that ATI might just have better drivers at the moment. Some will, no doubt, argue with me on that, but that's been my impression.
Since going to Crossfire, I can play BF2 at all the highest settings with frame rates around 100 FPS, dropping to no lower than 77 FPS. I was around 60 or 70 with one card, and it could drop as low as 30 at highest settings. So, no complaints from me here.
I may be misaddressing the SLI issue here, but it's worth a try. Is everything set up to use the correct card. Many multi-GPU motherboards still kind of have a "master" card slot, even though that concept has been somewhat done away with. If you're using the "slave" card as the primary one you're plugged into, that could be a related problem. What type of MOBO are you using? There could be some clues on that support site.
Good luck,
Frank