What CPU socket does your motherboard have? You can't install a socket AM2 CPU in a socket 939 board, & vice versa. If you don't have a board yet, you'll have to make that decision 1st (or at least the socket type). Once you've done that, you can search for a CPU in your price range.
There are a few different cores, plus there's wattage & stepping to consider too. AFAIK, the Brisbane core is the newest. Lower wattage is *usually* better (more efficient), higher stepping is *usually* better (lastest core revision). For example, if you have a choice between two 5000+ CPUs & one is 89W w/F2 stepping & the other is 65W w/F3 stepping, go with the 65W.
The higher end AM2 CPUs (6000+ & up) run at 125W, so if you go that route, take that into consideration when making your PSU choice.
This should help you to narrow things down:
http://products.amd.com/en-us/Deskt...