what graphics cards can be used in a dimension E520

My younger brother has a hand me down dimension E520 which can barely play most of the games he likes to play. I was thinking of helping him out with getting a new graphics card, I already gave him some of my old RAM when I upgraded and that helped him alot, however, I noticed that the PSU in the computer does not have the extra power connector required for most graphics cards. Currently it is running a Nvidia 7300 LE, are there any other graphics cards out there that will offer a performance boost without the need of buying a new PSU?