The best performance-per-dollar is really the E6600, if you can find it for around 350 dollars. I know TigerDirect.com has OEM versions of them for 359 bucks, which is a great deal since they're brand new. The models below this do not have the 4 MB of L2 Cache.
Also, make sure they are Core 2 Duos, not just Core Duos.