AGP is really an antique, and hasn't been supported by anything newer than three years ago's Radeon HD 4n00 cards, which are getting hard to find in game-capable quality (HD 4670). nVIDIA stopped adding new AGP cards back in 2006, six years ago, when Oblivion was still new.
Newegg had two HD 4670s with the correct GDDR3 VRAM, strictly for PCIe,
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102975
and
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102976
The only one of the AGPs that will work is three times that cost, at $113:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161337
Newegg had an HD 4650, which had roughly 35% less performance, but they were out of stock on that one.