I use good-old cheap XP1600's from Newegg. My main rig is daul XP1600 AROIA cores @ 1800 MHz (watercooled.) My file server at work is dual XP1800's, AGOIA stepping. They will do 1733 MHz, but it's not overclocked, since I can't afford to have system problems (servicing 50 users.) My wife' rig is dual Newegg XP1600's AGOIA stepping. They will do 1733 MHz, but at a noise level that I don't care for. I have them at 1533 MHz and 1.55 Vcore.
BTW, I use SVC GC68 heatsinks with Panaflo L1A fans (24 CFM @ 21 dBA) on the file server and my wife's computer. $6 for the heatsink, $7 for the fan. The heatsink comes with a fan rated for 32 CFM @ 28 dBA which is pretty quiet, but I'm incredibly picky about noise, so now they'er inaudible with the Panaflo's.
Robbie, you need an SMP-aware operating system like WinNT, Win2K, or Windows XP Pro (not Home Edition) to use both processors. As I said before, though, you're always using both processors when you game, you're just not using 100% of both processors. The only time you will use 100% of both processor is, either by running multiple applications, or by using a program that was programmed for SMP (even then, you probably won't be at 100% on both.) Adobe, for instance, programs all their stuff for SMP, so if you're using Adobe Photoshop or Adobe Premiere, you will use both processors.
Programs have to be written specifically for SMP because the program has to have instructions to send to both processors and it has to know when to expect data from the processors, and on and on and on. It's very complicated to program, so most companies don't do it.
If a program was not written for SMP, then it gets split between the processors. It's not always 50-50, sometime it's 70-30 or 60-40, but it gets split. It only gets as many CPU cycles as one of your processors.
As I said before, there are a FEW games that are programmed for SMP, but most (99%) aren't.