Egemence :
What is GPU? And how can I upgrade? And how many gb memory has the 9500 GT?
I know this thread is a bit old, but nobody seemed to answer his last question.
GPU means: Graphics Processing Unit,
The 9500 has something like half of a GB (512MB) to 1GB, I can't remember specifically.
As for an upgrade goes, I don't know what your power supply is, or how many PCI-E connectors you have, so I can't really make a recommendation without knowing.
To be honest, you should think about just building a new computer. Yours sounds pretty dated, and you won't get much more life out of it. The people on the forum can help you a great deal with putting together an inexpensive gaming rig, just ask.