There should be an AGP card installed in the first slot. Just pick up whatever AGP card fits your budget and make sure which ever one you choose works with a 300 watt power supply otherwise you will likely have to get a bigger one.
This is true. There is likely a single installed card (and maybe an old modem....) plugged into a brown slot that is offset differently than the PCI slots. That is the AGP slot.
If for some reason, your particularly variant only has PCI slots, then you can look here for PCI options:http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%20600007853&IsNodeId=1&bop=And&Order=PRICE&PageSize=20