There is no way to answer this, because it totally depends on the test you run on that hardware. What test you run (i.e. is it crysis, or vantage, or bc2, or gtaIV, or what?), and what settings you run that test at (what res, what AA level, what eye-candy settings, etc) will dictate how much your slower platform impacts your FPS.
In a general sense, crank up the res, the AA levels, and the eye-candy, and your 5850 will run just like anyone's 5850. It'll just be when the test is 'easy' for the gfx card that you'll see a performance hit from your proc being on the slower side.
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