What resolution do you play at? What games are you finding your system isn't cutting it as is?
I think in many games your cpu will hold back SLI 8800GT to a point it may not be worth it. I have found that overclocking my Q6600 from 2.4GHz to 3.0GHz boosted performance with two cards in Crysis, even at 1680x1050 2xaa/16xaf. In general, the higher the resolution you game at, and the more eye candy like fsaa you use, the more SLI will help. If you game at 1280x1024 on a 19" LCD, forget it. If you game at 1920x1200 on a 24" LCD, then it may benefit you. With that CPU, I would stick to one card and OC for added fps.