Hi! Welcome to the forums (as a member anyway
Hmm. Are you sure it's AGP and not PCI-E? I don't remember C2D ever being on AGP.
We'll assume you meant PCI-E. The best card that doesn't require an external power connector is the 6670. Can tackle games pretty well (don't expect to max BF3 but it could probably play it at medium haha) and is pretty cheap.http://www.newegg.com/Product/Product.aspx?Item=N82E16814131440
If that's overkill for your needs a 5570 would probably be a good deal.
Hope that gives you an answer. Cheers!