Basically is it worth the $300 difference to have Crossfired ATI HD5850s over Crossfired ATI HD5770? And what will the 256bit on the 4850 do over the 128bit on the 5770?
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It depends on what you want the 2 x HD5770 rivals/betters a HD5870 in performance.
Two 5850s would be faster. As for the value, it obviously fails an fps per dollar test, but if that were your concern you'd just be looking at a single 5770. It just depends on how much those extra fps are worth to you.
Two 5770's outperform a 5870 but leave you no easy upgrade options w/ two slots being taken.....of late I see many peeps going with twin 5850's or 470's .... one now and one at XMas time should get you at least through Xmas 2011.