Well, remember, you're going to pay extra for an SLI motherboard, so you should include that in your price differential. Spending $500+ takes you into 9800 GX2 range.
Spending $400+ on video gets into fairly exotic territory for me, so I can only point you towards the usual benchmarking sites. I don't see a lot of 8800 GTS SLI test, however, just 9600 GT and 8800 GT. I believe the 8800 GTS SLI outperforms the 8800 GT SLI by about 10 to 15 per cent.
I do have a suspicion, owing to the plummeting prices of 'midrange' enthusiast cards, that SLI might actually make sense at this point in the market and tech cycle. Personally, even if I could afford it, I'd probably shy away from multi-GPU setups, SLI or on-card, owing to their peculiarities, problematic multi-monitor support, etc. Unless you've a fever to play -Crysis- on the big screen, it seems to me one would do better to buy an 8800 GTS now and see what the next generation brings over the summer and afterwards, as we finally move beyond G80/G92 technology.
Anyhow, a more in-depth discussion of the merits of SLI is perhaps better taken to the video and graphics forum.