The best bang for your buck is an intel Core 2 duo (maybe e8400) or cheap intel quad core (around 2.4ghz) and overclock the thing up to 4ghz with a decent cooler.
Nah, I usually don't have processors waiting around to be used....well good ones anyways. lol and im talking about when dinosaurs roamed the Earth.....A 1 core Intel processor at 1.1-1.9 GHz
I have to agree with JCB, yeah you might get a cheap one on Ebay but idk about buying hardware from Ebay....well that is just me I have a phobia of Ebay sometimes.
I have a E4600 Core2Duo, @2.4GHz.
thou, unsure if it works. haven't had time to test it.
I'm in Canada. so its probably not worth the time and money anyway.