Are you in the US? If so check this 512mb GDDR3 HD4650 AGP card from TigerDirect for $70. http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=6438898&CatId=3670
It has 1600mhz effective memory it is closer to an HD 4670's 2ghz memory clock vs the reference HD 4650's 1ghz memory clock. After a little OverClocking you should get HD 4670 performance in AGP
If you head over to Anandtech or HardForum you might be able to get an HD 3850 AGP card for $50 - $70 shipped used nd they would be faster than the HD 4650 without OverClocking.