Like has been said, both motherboards will perform equally well at the same clock speeds. The 790GX is the far superior board for overclocking and has another slot available for CrossFire, but since you don't need any of these features, just go with the Gigabyte board. You'll still be able to get a small overclock on there, though.
This should be a HUGE upgrade from your current CPU.
Edit: I see you say you already have 1066MHz RAM. Is this DDR2 or DDR3? Because if it's DDR2, then you're out of luck with both of those motherboards.
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