How on earth did you wind up with an AGP slot on a recently built computer? AGP motherboards haven't been manufactured for a few years, although plenty are still in circulation. The GeForce FX 5500... well, that was around before the Pyramids were built. You really should return that artifact to Indiana Jones, asap ;)
In any case, The best AGP card money can buy is the Radeon 3850. It's slightly faster than the AGP Radeon 4670 but has two drawbacks. It's very hard to find nowadays and requires a 450W power supply. It usually sold for around $90 from NewEgg, Tiger Direct etc. There are a few left on Amazon.
http://www.amazon.com/Sapphire-Radeon-HD3850-PCI-Express-Graphics/dp/B000YNUX3W/ref=sr_1_2?ie=UTF8&s=electronics&qid=1274462442&sr=8-2-spell
The next-best card is the AGP Radeon 4670, which is newer and more energy-efficient (a 400W power supply is ATI's official recommendation but that's an inflated estimate, it usually runs fine on 350W). That card sells for about $120.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161337
The Radeon 4650 is the 3rd-best card you can buy (about half the speed of the previous two) but it's only $75ish and uses even less power than a 4670.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150433
Nvidia abandoned the AGP interface several years back when they discontinued the GeForce 7900GS. They never manufactured any 8000 series, 9000 series, or 200 series cards in AGP- so they aren't competitive in that arena.