1) the $79 WinXP at Office Depot is probaby Home Edition, Upgrade version, is that correct? You'd need to use your Win2000 Pro as the qualifying previous version to use an Upgrade, and unfortunately you cannot upgrade from Win2000 Pro to WinXP Home:
WinXP Home Edition upgrade matrix at Microsoft.com WinXP
Pro Upgrade could work, however. Bottom line, using an Upgrade version ties up your Win2000 license and limits you to WinXP Professional Edition.
If they're selling retail
full version WinXP, either variety, for $79, then please post a link to it!
2) if dual-core is in your future, WinXP offers proper Hyperthreading support. As I understand it, dual-core A64s mimic Hyperthreading so that WinXP Home can get at both cores (XP Home supports HT's virtual CPUs, but not dual physical processors).
3) if you have a fast connection, you could download a 180-day trial of WinXP Pro x64 Edition from
Microsoft's site too. Been there, done that, decided to pick up retail full-version 32-bit instead for this go-around.
Hope that helps