A few things to keep in mind:
Windows XP Home and Media Center Editions are not SMP (means Symetric MultiProcessing, basically dual-cpu) capable.
If you have XP Pro, Linux, etc. keep in mind that your applications may not support SMP. Most don't. So they will run on one proc or the other. Now your OS will try to load balance your running apps and services so that one CPU isn't fully loaded while the other is doing nothing, but you need to know some tricks to assign them specifically.
Most current games do not support SMP although most will run on an SMP platform just fine.
The Intel Hyperthreading scheme is a bit different. You will see 2 CPU's in Task Manager but HT simply handles multiple threads of an app using the same processor IOW it creates a virtual processor. WinXP knows this and handles it differently than a dual-CPU system in that you can use HT in XP Home, etc but not SMP.
