AGP is an older, outdated slot for video cards, they have not came out with new or high performance cards for the AGP slot for several years. A 7950 is a PCIe slotted card, the standard slot used in video cards for nearly 10 years now.
Even so, the rest of the system is slow and outdated as well, and even if you do find a decent older AGP card for this system, it is not going to help you much at all.
In short, if this machine is not performing well enough for you, you need a new machine.
No matter what you do or how much you spend, you are going to have a slow outdated machine, sorry but that is the truth of the matter.