You need to replace that PSU. At best, it works fine, but is an older, inefficient design that probably also does not provide enough of its amperage on the +12V rail(s) where it is needed by a modern PC. At worst, it will overload and croak, taking your mobo and/or other parts with it.
You don't need a powerful one, but you need a quality one. If the video card you decide to get does not need auxilliary power from the PSU (up to an ATI 4670 or nVidia 9500GT or "low-power" 9600GT), an Antec Earthwatts 380 would be sufficient. Use a 500W model for more demanding single video cards. If you cannot get, or don't want Antec, other good brands include Corsair, PC Power & Cooling, Seasonic, and Enermax.