While you can have a PCI card, you're confusing PCI with PCI-Express, which the 8800 series uses (and above).
While in order to use a PCI-Express card you will need a new motherboard, CPU, and RAM, you might be able to find a graphics card that is decent for AGP yet.
However, since your computer does have AGP, your CPU may need to be upgraded to use the latest games at a decent framerate.
Do you know the specifications of your PC? Include the Power Supply specifications that are on the Power Supply.