In terms of power the 4850x2 would be a better choice for around the same price. I came across this issue when the 4850x2 was around the $300+ mark and opted for the xfx gtx260 216 black edition because it's a great card for $245AR if you factor in the bundled software (farcry 2 + 3dmark vantage) and the double lifetime warranty. My upgrades went from the ati9600xt--> 6600gt SLI --> gtx260 core216, and my 9600xt is still able to play tf2... SO if you're like me and tend to upgrade slowly (4-5 years?), go with the gtx260+. Atleast if anything should ever go wrong, you'll be covered for life... who knows how long the sapphire will last for. On the other hand, if you upgrade occasionally and want higher performance, the dual GPU 4850x2 should be your pick.
I prefer Single GPU cards since some games don't use SLI/Crossfire (though hopefully that will improve) however being stuck with one 4850 isn't that bad, and when the X2 works it should beat the 260. Just something to consider.
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