I dont know anything about the x600pro, but thats a PCI-E mobo, so your graphics card might require the powerinput from a 24pin PSU. If the gfx card does require that extra power and since your using a 20pin PSU, that would probably cause problems... im assuming.
If the x600pro has a PCX power terminal on it, you dont neccessarily need a 24pin PSU. you can just get a Molex to PCI-E power adapter.
Most decent 24pin PSU's have 2x 12v rails it think. so if you use your old 20pin PSU, it needs to have a decent strong +12v rail. probably at the very least +12v@20a .
your probably using the NTFS file system. thats the default one XP usess. just go to /my computer/ "right click" Harddrive "go to propertys" (it tells you the file system there.