basically, your options are 3650 or 4650. 3650 would probably be plenty with that procesor. 4650 is a good bit faster, but whether it'd make music difference in that PC, i can't say.
nvidia doesn't really have any recent agp offerings, afaik. i think the 8600's were the last agp cards.
used is a very good option. retail markup on new AGP cards often has them costing 50% more than their PCIE counterparts.
edit- 1650 works, too, probably pretty comparable to the 3650.