With a rebate, it's possible to get a 4870 for under $100 (usually closer to $90). Based on all of the reviews I can find, that will put you somwhere between a 5850 and a 5870, which retail for way more than $200. It also is TONS faster than the $200 Nvidia 460 (somewhere between a 470 and a 480).
Am I missing something or is this a great price/performance spot right now if you are willing to deal with crossfire and rebates?
Crossfire actually might be an advantage for some people because you can run 3 monitors with it while you aren't gaming by disabling crossfire and using the 2nd card.
4870 is a great card. reason it is cheaper is its older technology no dx11 support. performance is less than the 58xx series and a 460 for some applicaitons (in most cases 5800 would be better, but the 460 in dx11 capacity as 4870 does nto support it) but its hard to beat in performance from a value standpoint and most games still have a dx9 or dx 10.1 mode
2x 4870 are more powerful than a single 5850 but you get more heat and an increased power usage. You might want to see how the new 6xxx series affect the 5xxx series price and buy a single 5850 and buy another one later when the prices go even lower.
That's if you want to buy and it also depends on what resolution do you intend to play the games.