What are you wanting to do with your system? and what mobo do you plan on using? Ram? if you provide a little more info we can help you out a lot better and give you some better, more well informed suggestions. because I would suggest a dual core with 1GB of ram, but...
if that's the case then I would suggest the 3700 and 1GB... No use paying for the extra .2 Ghz when you can OC it and get it from the 3700. But if you can spare the cash, then I really would suggest the dual core 3800.