Does performance matter? If it does, then pick up the 4670 AGP. It's an AGP 8x part with 512 or 1GB RAM. They're running $110-$140 on Newegg right now, and some provide newer interfaces like HDMI and DisplayPort. If performance does not matter, then I would hit up eBay or a local shop for something cheaper. Odds are good you can find an old cheap machine w/an AGP card (or maybe just a card) on craigslist locally for pretty cheap.
P.S. - 4670 AGP will require a 4-pin molex connector and an additional 75 Watts of juice to run. As far as I can tell, the 4650 won't but won't be as fast either.