While dual or quad cores are not necessary, there's rarely a time where there's less than 25 services or apps running on your computer, just because they're not all actual programs, doesn't mean they're not there. Antivirus, messenger, firewall software, web browsing, etc.
I recently upgraded from a dual core to a quad core, and there was a noticible difference when it comes to gaming and ripping dvd's.
If you're building your own box, I'd suggest the Intel Quad Core Q6600, 2GB of OCZ DDR2 800 ram, a 320+gb Seagate 7200.10(SATA II) hard drive, an Intel or Asus motherboard using the Intel P35 chipset, a good power supply from someone like OCZ, Corsair or PC Power and cooling, and whichever video card you deem necessary.
If you're going 8600GTS, then a powersupply somewhere around the 500w mark should be fine, if you step up to the 8800 series, then I'd bump it up to 600w. You can get a nice case from Antec, the P180 series are great, lots of airflow, good big quiet fans and lots of room to work inside.
At this point, I'd probably go with Windows Vista on a new system, and with a dual or quad core cpu, 2gb of ram and a good video card, it will be just as fast or faster than xp with older hardware.
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www.ncix.com. Their service, speed and pricing have all been top notch.