Vista vs. XP: The correct answer in this situation is always XP, no matter the circumstances, unless the question is "which OS is more effective at making my computer unuseable?". Professional is better than Home because Home can't deal with multiple CPU cores very well.
Dual vs. quad: If you were using this system for SETI@home, Folding@home, Prime95, CAD rendering, digital video compression, webserving, etc., then you'd want to go with quad. When it comes to games, however, you're better off with a faster-clocked dual-core system.
ATI vs. nVidia: In the previous two generations of graphics cards ("Radeon HD 2k vs. GeForce 8k" and "Radeon HD 3k vs. GeForce 9k"), green team (nVidia) has soundly thrashed red team (ATI). In the current generation, however, ATI seems to have finally gotten their sh!t together, and is now getting nVidia's nose more than a bit bloody. No matter who holds the performance crown, though, WoW does not require a very high-end graphics card and neither will Warhammer, so you're probably looking at either the HD 4850 or GTX 260. Given that the ATI part is about $100 cheaper, that's the one I'd pick. As for manufacturers, I absolutely love Sapphire. They make high-quality stuff at insanely low prices.
Intel vs. AMD: Again, if you were using this system for SETI@home, Folding@home, Prime95, CAD rendering, digital video compression, webserving, etc., Intel easily wins this round. Games, however, especially high-end ones like Crysis and Unreal Tournament 3, are more taxing on the video card(s) than on anything else. I suspect that they will run flawlessly even on a humble Athlon 64 X2 5400+ Black Edition, though if anyone has the benchmarks to prove me wrong, they are free to do so. And if this suspicion is wrong, and if you plan on playing such games in the first place, then my next suggestion would be the 3.0 ghz Wolfdale (E8400), which will chew up Crysis, spit it out, and piss on its grave. Note: if you go with Intel, be sure to get a motherboard that supports DDR2-1066. There's no excuse for limiting yourself to DDR2-800, and DDR3 is overpriced jank.
Asus, once the go-to company for motherboards, seems to be getting complacent and incompetent as of late. I'll let you know when a real leader re-emerges, but the current companies vying for the throne seem to be MSI, Gigabyte, and Biostar. You could also take the path less traveled and go with Tyan; they're not known for gaming, but their track record of reliability is unmatched.
I feel the need to pimp out Excelstor as the up-and-coming king of hard drive companies. They are offering very good drives at extremely aggressive prices. You can get 500GB of SATA RAID goodness for ~$100.
