if putting bottleneck into account. i think 5670 or 5570 is the better choice, lower power consumption and better tech. he had already got a very weak PSU. I strongly suggest changing the PSU too. I assuming that subline want as many power as he can get from his old pc.
bluescreen is right, assuming using single core P4 with HT. you won't go very far in extreme gaming.
one problem, if you are using onboard all along and machine that old. i wonder how long the mother board (in specific, the capacitor on the mobo.) will survive if the power load skyrocket with the new GPU.