Lordofdeath24 is right. Your PSU seems to be only 220 Watts, which is okay for a single HD7750 provided the computer is not too loaded (and I suspect it is not, since it is a slim factor and you can only put so much inside).
If you pick up a HD7750, be SURE to get the one released earlier (<900Mhz core clock speed) without a 6 PIn PCI-E power cable requirement.