it's not easy to decide between a x800xl and a 6800gt. they're just about identical in both performance, cost, and image quality, so i suggest you get whichever is the cheapest, or has the bundle you're interested in. none of them are super expensive though, and should give you a very nice fps boost.

another reason to wait until next year is that amd is releasing socket m2, which will replace socket 939. your system is pretty decent though, so if you upgrade to something like a 6800gt now it should last until dx10 and socket m2 is out, at which point you can sell your current stuff and go all out

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