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?
thanks for the response, i know its an old system ( in pc years ) but i dont do a whole lot on it ... my psu died and i just need a replacement - mild gaming, internet, some school work. so i should be fine
There are other good choices, but just to keep it simple, check out a 430W or 500W Antec Earthwatts.
I'd also suggest that you'd see a notable performance improvement if you upgrade your GPU, even to a HD4670 (ATi) or GT240 (nVidia).