I would not throw ANY kind of serious money into a video card right now - wait for DX10.
SLI is not economical at all - one high end card will spank 2 lower end cards in SLI.
If you are at all budget restricted, don't spend a premium for components that perform marginally better than the next step down - example, the E6700 is not worth the premium over a E6600. Same with that high end ram - the standard XMS2 DDR2-800 CL5 is plenty good.
I'd personally get a single slot board with the P965 chipset - such as an Asus P5B-E - and a "placeholder" video card - such as a 7900GS or even one of the 7600 series - and update to a high end DX10 card down the road when the dust clears. I would NOT toss any kind of serious bucks into DX9. Yes, the 8800's support DX10, but it's the first one and it's too early to trust them to properly support it. Same deal with the OS - just get XP Home for now - with an upgrade coupon for Vista. The 680i chipset shows a lot of promise, but it does have issues.