I agree with JeremyS on that one. If you're looking to use the vast majority of your existing hardware, you should have no real problem hitting the $500 mark or so. If you're looking to go cheap, you can buy some of the older 64 bit processors, but i think it would be well worth your dollars to invest in some of the higher level processors, for example:
http://www.pricegrabber.com/search_g...sterid=1905397
Any time you can get a meg of L2 cache you're going to see awesome performance increases.