The Pentium Ds run very hot. And if you haven't cleaned out the HSF etc in a while, it could be thermal throttling slowing your PC down. Download this program: http://www.cpuid.com/hwmonitor.php
Take a screenie and post it up here.
What is your budget? And you can certainly get a lot for your money with an AM2 platform. And what are the rest of your specs? Do you have DDR or DDR2 RAM?