Video memory is noting
The performance of video card depends on the GPU, not on video memory
For, example 7900GS 256MB is much faster than 7600GS 512MB
And two video cards in crossfire (or SLI) will not give you twice the performance. It will just give you about 40%-50% increase, so that why I think it is waste of money. You pay double the price and don't get twice the performance. And two video cards in Crossfire would consume a lot power (specially if you get two X1950XT)
If I were you I would buy 8800GTS 320MB then I would upgrade later to 9800 series next year. You don't need to buy 2 video cards. Just get one strong video card.......
And this is the 8800GTS that I recommend
Newegg.com - XFX PVT80GGHF4 GeForce 8800GTS 320MB 320-bit GDDR3 PCI Express x16 HDCP Video Card - Retail
Single simply gets beaten by dual.
Not always true
7900GS beats 7600GS in SLI
8800GTX beats 7900GTX in SLI