Why wouldn't you step up from a 9800gx2? It'd be cheaper to pay ~$150 for a GTX 280 than ~$300 for a 4870, right?
Actually reading all the reviews, I would rather keep this 9800gx2. It seems to perform better than the 280gtx on the majority of the tests while saving me $150. I may end up getting a waterblock to cool it and overclock it to milk more performance out of it.
Besides if I purchase the 4870, i can sell this card for $400 and make $100.
So step up: costs $150. Sell 9800gx2 and buy 4870: make $100. That is a $250 swing.