GPU for brand new build: Go SLI or Not?

I'm building my first homebuilt PC, and I'm not certain whether I want to go with the SLI capability or not. My original plan was to get 1 8800GTS 640MB now and then get a second later (in a couple of months) and run them in SLI. But now I'm thinking maybe just going with 1 8800GTX now and abandon the SLI capability altogether (get a P965 or P35 mobo instead of 680i). The PC will be used mostly for gaming, and I think the one GTX will perform better than 2 GTS in SLI (especially since I won't have the 2nd GTS for a couple of months).

My other thought is that when I want to get the second card, the next gen will be here (in time for Christmas). I'm almost certain one of those will outdo any GTS SLI setup. I'm thinking that SLI is really only worth it if you can afford to get both cards at the same time, so you're not trying to play catch-up with technology.

Also, I'm planning on installing Windows XP SP2 to start, with the intention of going to Vista later, when there are fewer problems and more DX10 games. I appreciate any thoughts/advice/suggestions. Thanks!