That's a really old system. A lot of effort to upgrade it would likely be wasted. Still, a PSU is something you can buy and plan on re-using when you build a new rig. A quality 500W-550W PSU can handle almost any single graphics card (just not the HD5970 or GTX470&480). Look for full range active PFC (no little voltage switch) and 80+ certification. Antec, Corsair, Seasonic, and Enermax are among the better brands.
What do you intend to do with this machine?