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IMHO the E6700 is still a great chip. Few games currently make use of Quad core, so the upgrade to a Quad really wont get you that far. 4GB ram, E6700.. Your system can easily handle any of the latest graphics cards, and thats the easiest upgrade that will be noticable when gaming. Or you could wait until the end of the year, or early 2009, and pick up a Nehelam system. The Nehalem CPU is looking quite promising.
I think an ATI 4870 would be a good choice, and likely the best bang/buck upgrade you could make at this time.
At the moment for gaming Quad's mostly offer futureproofing, but with the better implmentation of Quad + Hyperthreading on Nehalem, I wouldnt bother building a "future proofed" rig right now.
Plus, if you were to get the graphics card, and then found your CPU was holding back the system, you could clock the cpu anyway.
, I wouldnt bother building a "future proofed" rig right now.