The 4870X2 has the same problems as it has two graphics cores, a lot of games are not optimised for dual graphics cores and therefore it suffers the same problems as any other Crossfire/SLI solution if the game isnt optimised for Crossfire it will run slightly slower than the GTX280, but if it has full support (COD4, WIC etc) it will be nearly twice as fast as the GTX280
i own the single core 4870 and its really fast, for 9/10 games you wont need the extra speed unless you have a huge monitor like 30" or something
Hopefully this will help you for more info check out these reviews
http://www.hardwarecanucks.com/forum...rd-review.html
http://www.anandtech.com/video/showdoc.aspx?i=3354&p=1