A single-core cpu will run games faster than an equally priced dual core cpu... At least for now. A dual core A64 X2 will run you a minimum of $500-$700, but you can get a single core cpu that runs 200-300 mhz faster for the same price, and the faster core speed means it will run single applications faster.
For gaming, get single core now. Maybe later on the dual core cpus will be better suited for gaming but they're not the fastest right now.
That's not to say the dual core cpus aren't great and also very fast, but AH2 certainly won't benefit from a dual core cpu right now so if you're going to spend that much money on the cpu, get a faster single core one. Either a fast San Diego core, or an FX. The socket 939 cpus will be around for a while so there'll be plenty of time to upgrade to dual core later on when the price drops. That's why I would cap your cpu price at $400 or less... Save the money you'd spend to get the absolute fastest and use it for an upgrade cpu in a year or so. By then, another $300-$400 will get you much more than spending $800 now would.
Aim for the 90% speed because it costs half as much as the absolute fastest, and spend that other half a year later when it'll buy you waaaay more than the 10% it would have gotten you if you spent it today. That advice goes for video cards, cpus, and hard drives. Motherboards are commodity items so spend whatever it takes to get the mobo features you want because they're all within $30ish of each other. But you can get 90-95% speed on a vid card and spend $450 instead of $600, get 90% on the cpu and spend $400 instead of $1100, and get a 300 gig hard drive for $130 instead of spending $250-$300 for a 400 gig drive. Then buy what is the 90% solution a year from now, and it'll probably net you a 30 to 50 percent speedup over what's available now, especially in the vid card.
If I had unlimited cash, I'd go for the 7800GTX because the 7800GT is one of those "crippled" versions and I hate those. If they released a lower clocked 7800GTX that had the full number of pipelines instead of lower clock AND fewer pipelines, then I'd say get that one, since that's essentially what they did with the GF4-4200 and 6800GT. But since nvidia isn't offering that sort of enthusiast option this time around, I'd say either save the money and go for a 6800GT or go all out and get a 7800GTX. Of course, I'm probably just being unreasonable because the 7800GT is a very very fast video card right now even with the reduced number of pipelines and some of them overclock rather well so you can get back up to the same GPU speed, just with a couple of fewer pipelines. According to many benchmarks using today's games, the 7800GT and 7800GTX post nearly indentical scores so you won't miss much getting a 7800GT but you'll save $100-$150 vs. a 7800GTX.
