You are better off getting the 8800GTS and overclocking it instead of getting 2 8600GTS, they just don't pack as much power as the 8800GTS.
Check this comparison. The 8600 in SLI just wins a bit from the 8800 (so much, you would hardly notice a difference) But the 8800 has much more Shader possibilities (an essential in DX10). And if you overclock the 8800, it will get much more powerfull. If you overclock the 8600 SLI, you would gain more power, but not more in shading power. Plus, I think there is a higher chance that you overheat your cards in SLI if they are close too eachother.
My advice is get the 8800GTS 640mb. And you could always save more money to get the 8800GTS in SLI later, which would be much better than the 8600 in SLI.
Downside on the 8800 is that the 8600 is newer, meaning that it has a different video processor the 8800 series has not. If you are planning on watching a lot of HD movies later, you might want to reconsider the 8600. If you plan to do a lot of gaming later, get the 8800.
You also didn't mention the inefficiency's that SLI has, you will never get 2x the performance just by adding in another card.
As I recall, the 8600GT is only slightly faster than the 7600GT? (not sure on that one).
The 8600GT is twice as powerful as the 7600GT, the 8600GT is almost as powerful as the 7800GT. By that I really mean
slightly weaker.