The 3.06 533 fsb p4 is the fastest you can use. Starmicro has one for $27.50. You can find the link on pricewatch under cpu>p4 3.06. Be sure to get the desktop version; they also sell a mobile version, which may not work with your board or heatsink. You will notice maybe a 20% performance gain, especially with 1-2 gb of memory.