I seriously doubt I'd notice it. The performance difference is a full 13FPS in Crysis from the 4870 to the 9800GX2 with 0 price difference. If it's really a problem, I'll just return it to newegg, they're really good about those kinds of things. And if the 4870X2 comes out and rips apart the 9800GX2, I could just return it.
And with newer and newer drivers, I'd assume they'll minimize it even more.
I'm guessing I'll do almost a whole rebuild of my computer. Swap out the motherboard for my P35 board, upgrade the video card, upgrade the PSU, and switch to Vista once and for all now that I'll have a DX10 card.