I upgraded my compaq pc which had the same hp 250w to an antec 350w atx power supply. Set a budget; if upgrading the video card, factor in it's requirements also before ordering. A premium card such as the 9800gtx needs a 600-700 watt quality ps, such as antec, corsair, seasonic, enermax, or pc power and cooling. A mid range card, such as the 4850, would do fine with 450 watts.