Lets see, you, or someone you know, bought a "value" system with no AGP slot and Integrated Intel Extreme, right?
The top dogs as far as PCI graphics cards are GF FX5600, Radeon 9100, Radeon 9200 and FX5200. All will perform fairly similar with the first two taking honors by a little. The FX5600 is costly and rare, so finding a R9100 might be the best. More likely you will have to settle for a R9200 or FX5200. None will wow you, but will be far better than Intel Extreme.
MSI K8N Neo2 platinum, A64 3000+, 1GB Corsair XMS 4000 Pro Series, Radeon 9800 Pro