My general rule of thumb
Computers < 1000 mhz I go w/ Windows 2000 Pro because it's more streamlined and contains less "fluff" that XP has. Therefore your system will overall run faster without the extra burden, and it has the same capabilities as XP.
Computers > 1000mhz I go w/ Windows XP mainly because the processor can easily handle the extra fluff and can take advantage to the capabilities that XP has over 2000
Computers < 1000 mhz I go w/ Windows 2000 Pro because it's more streamlined and contains less "fluff" that XP has. Therefore your system will overall run faster without the extra burden, and it has the same capabilities as XP.
Computers > 1000mhz I go w/ Windows XP mainly because the processor can easily handle the extra fluff and can take advantage to the capabilities that XP has over 2000
