You will be wanting to go dual core. That leaves you with the AMD Opteron or the AMD X2 both are very good. A entry level X2 and Opteron should be around 300 USD but these in their own rights are fast. I suggest you learn about Dual Core Processors and go to newegg and start searching for various Opteron Processors and X2 Processors.
The Opt 165 is a fine processor but have a personal prefrence to the X2 Series. The 165 can be OCed pretty far you shouldn't worry about that many members have bought this an OC'd it I'd ask around.
You should definltey get a dual core, the Opty's 170 out right now, the week 46, OC pretty darn well, im seeing people get around 2.8 ish, though i cant determine what OC you will get.