now cost allmost the same 260 for the 4850x2 and 240 for the 4890, but I heard that the x2 have a few problems with the drivers soo. I go for the x2 or I buy for allmost he same money a video card a lot less fast???
I have a phenom II x4 920 and aPSU coolermaster extremepower 600W
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depends what games you play. some games don't like multi-gpus... make sure the games you play aren't one of them.
to be safe, go with the 4890.
I usually play cod4 and cod5, pes2009, world in conflict, crysis and gtr2. Thats game have support multigpu???