I have a similar rig: 3500+ OCed to 2.7, 2 GB ram, 7800 GT, X-Fi Fatality
I used to have 2 7800 GTs in SLI but i got little improvement.
A 7800GT is kicked in the pants by COD2 and FEAR, unfortunately. Personally, I can play FEAR at all max BUT @ 1024x768 and 0xAA (16xAF is fine)
With SLI I could up the AA to 4x and that was about it...
COD2 is poorly optimized so DX7 is highly recommended. 2 7800 GTs in SLI DX 9 is fine cept body count set to normal and world smoothing only.
BF2, I run great with 1 7800 GT @ 1280x960 4xAA and 16xAF