You don't really need a new CPU. The difference between 1.3 and 1.5 GHZ is minimal at best. The one you have will work fine. Upgrading the RAM to 512 MB is a good idea though. Also, don't get Best Buy to charge you for RAM installation when you can do that yourself in a matter of seconds, or even get someone you know to do it for you for free.
Minimum system requirements for XP Home/Professional
Minimum system requirements for XP Home/Professional