For a card on the cheap that will play most new games.. the 6600GT is what I'd reccomend. The fx5700 you've got listed there wont keep up with the new games. its around $200, but can be found about $180.
And yeah. to find out if u have an agp slot, look above ur PCI slots and if theres a brown looking pci slot thats kinda outa place.. then you've got one. if there is any writing on the slot, then its most likely onboard graphics, and you wont have an AGP slot
Sig reinstated. SigSanta