SLI does not deliver a 100% performance boost. It gives about 10-40% performance boost, depending on the application. It is definitely not cost effective for you to go SLI. And besides, you would be robbing yourself of the ability to play DirectX 10 games (the only NVIDIA cards that support DirectX 10 are the 8 series cards).
So, basically, here's what I'd do:
If you are really dying for an upgrade right now, I'd go with the 8800GTS 640mb edition. 320mb just isn't going to be enough for the newest games. However, if you can wait a bit (say, till November), you could get the new 9 series cards. The 9800 is rumored to have twice the power of a 8800, while being smaller, cooler, and cheaper (due to it being built on a 65nm process).
Or, if you go with EVGA you can get the best of both worlds, due to their step-up program. It allows you to return a card purchased within the last 90 days and put the cost of it towards a new card. So, you could get the 8800GTS now and then get a super cheap 9800 in November.