Yes, I'd have to say the XP1800+.
Use
pricegrabber.com.
Right now a retail boxed 1800+ is going for $84.
I have a friend who has his overclocked to 2100+.
If you want to pay $27 more get a 1900+ for $111 (retail of course) and overclock it to 2200+.
The reason I am only giving you retail prices is #1 those are the best to get because you get a warranty and #2 the price between the retail/oem is not significant enough right now IMHO.
If you want suggestions on motherboards, ram, heatsinks and fans, etc. just tell me so.