Funny no one's mentioned AMD systems. An AthlonFX-53 is pretty fast and not that expensive, plus you can bump the clock speed a touch with no problems. But then if you have 3K in you pocket, you could buy a decent dual Opteron system that shouldn't have any problems doing Doom3 with all the goodies turned on.
I used to use an AMD chip, but left them when Germany began stabbing us in the back.
An FX-53 is about $800, with the 939 pin being a tad more. If I am going to spend that kind of cash, I will dump $200 more in and get the 3.4EE with the 2 MB level 3 cache, in the 775 pin form(the 925X chipset, when available).
I might look at AMD's 64 bit systems when Windows 64 comes out in retail version (its only a beta right now)though I'm not sure the 64 bit is going to be as hot as some people say.
I still think I am going to hold off on the next Gaming Box. The PCI Express X16 interface for video cards may just be worth the wait.