Get the GDDR5 version of the HD7750. The DDR3 versions are underperforming abominations.
If your system could run a 9800GT, it will have no trouble with a HD7750.
A "480W" PSU sets off a little warning light in my head though; it brings to mind Logisys, which is a PSU-shaped object. It is not safe to be attached to computer equipment if there is any possibility that it will be plugged into an A/C power source at the same time. It may have enough wattage (250W would be enough for a HD7750), but is missing "optional" components like line filters, such that it could die at any time from a random power disturbance, possibly killing attached parts.