I'd go with a 6600GT.
U could also get a 6800GT, which would be better, might be a little over ur budget, but u can check that out yourseld. The 6800GT is the best u can get for AGP port.
But the reason why I'd go with the 6600GT is that the cpu is old, so it will be bottlenecking the card so u might not notice a big difference between the 6600GT and the 6800GT.
And just check that the mobo has an AGP port, if it only has a pci port then it's time for a new mobo. And how much wattage does ur power supply have? I think the 6600GT needs maybe 300W and the 6800GT needs maybe a 350W. Not sure if those wattages are correct, but it's something like that. U can check them from google if u want to.