It may seem kind of harsh, but if you're trying to overclock the FX 5200 for performance reasons, really... don't bother. There's very little you can do to that card to make it worth your time. If it's just to experiment, have a blast.
Assuming it's AGP (I keep an old FX 5200 PCI card to test for bad video cards), try looking around for a used 6600 / 6600GT AGP card. They should be decently cheap. Stay away from anything like the 6200 if you're going to run games though. Like the 5200, it's definately not a 3D-monster.