I run a i7 920 system with 12GB ram. For video cards I use 2 260GTXs in SLI mode, haven't had any issues, and I did see a FPS difference between 1 and 2 cards.

Note that I overclocked both the CPU (moved it to 3.8Ghz, and am not using stock cooling), and overclocked the video cards (turned fans to 100% on both, temps around 55C).

Honestly though I saw the bigger difference with symbolic linked directories and fast hard drives, I run 2 300GB velociraptors (10K RPM) in Raid 0 mode.

Don't forget to back up important data, when that raid fails, you won't be able to recover without a backup.