i seem to get alot of lag while i play COD4 with my hd 5850. i left the "optimal" settings on the only thing changed was the resolution to 1680x1050( its my monitor native resolutoin) any idea why? i didn't get this when i had my hd4870. i am using a core i7 860, evga p55 sli mobo, 4 gigs ddr3 (@2000Mhs) and a 550W rocketfish PSU.
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Have you tried installing the latest Catalyst 10.3 drivers?
wondering why you skimped on the psu when you got parts from higher end manufacturers.. haha.. check the drivers.. and check if psu is delivering