I think the HD 4650 is the better option because your CPU is a bit slow compared to more recent CPUs and will hold back the HD 4670's performance. Just like PCI video cards, AGP video cards are no longer being produced.
Your next upgrade (after buying the HD 46xx) will have to be a new computer.
If you are into getting Nvidia as only a matter of preference then you can try to hunt for a 7800gs but beyond that you have little choice but to get an ATI card and the card every one is trying to get you to buy is about one of the few decent options for AGP that aren't crap. If for just 2D and video ect then go ahead with the 6200 but don't expect any gaming.