Well, if you're not overclocking, you won't see much of an improvement between any of those processor. The socket 939 platform offers dual channel memory operation. THEORETICALLY, it doubles memory bandwidth, but it doesn't. If you can afford a socket 939 setup, I'd go for it! It'll be highly upgradable to any dual core processor made by AMD right now, (excluding socket 940 opterons) and you'll be good for a while. If those are out of your price range, you won't be disappointed with a socket 754 Athlon. Gameplay will still be very good at this point. Don't worry. The 6600GT is a pretty good card and you'll like the way it looks on most games.
But then on the flipside, there's a whole other situation brewing considering your predicament.
If you were to buy a socket 939 board and socket 939 processor, the motherboard would have to support AGP for your 6600GT. So that kinda nukes the entire upgradability thing if you ever want to update your video card. So... It's really up to how much you want to spend.
<b>I'm an unhyphenated American.</b>
Intel Q6600 @ 3200 1.4v
Abit IP35 Pro "The Snake"
2x2gb A-Data @ 800
Diamond HD 3870 512mb
Great FORD TRUCK