I just asked a similer question about upgrading my friends 4800+. Turns out that it's not really worth it for any quad core upgrade on the AM2 motherboards. Try to get a 3.1Ghz 6000+ or 6400+. It seems worth it to me to go up 600-700Mhz for ~$75, but it's up to you. You may even be able to find a cheap used one with all these people going to the Phenom II's.