Do you know the model of your motherboard? If not, you'll need to open it up and have a quick look.
Link #1
That green one is an AGP slot
Link #2
Those are PCI-e slots
Their isn't all the much difference between the two, just be sure which one you have before you order coz if you get the wrong one, it'll be a complete waste of time!
If you're PC is older than a year then I'm pretty much 100% sure it'll be AGp, anyway. If you're rig is really quite old, you may be better off getting something like the 6600gt because your CPU, RAM, etc. will just bottleneck the card anyway and you won't get full performance.
Good luck, though!