What kind of graphics card will work for my desktop?

I have an old Compaq desktop that still runs pretty nice. I tried to run the minecraft demo on it but it showed an error message saying that it could run it. I know it needs a new graphics card and I know that I gotta use one that works with the wattage of my computer. Here are some specs:
Desktop: Compaq presario SR2020NX
AMD athlon 64 processor 2.2 GHz
4gb ram
graphics card: Nvidia GeForce 6150 LE

What psu do you have and what is the wattage also can you not
Locate a PCI or PCI-e slot on your motherboard?