Well, even if it's overrated by 20%, it still ought to handle a HD7770. Of course, overrating is not the only problem common to cheap PSU-shaped objects. It may have sorely deficient filtering (input and/or output), significantly affecting how "clean" the output is. Dirty power can damage attached components. It's your call of course, but getting a quality (high [claimed] wattage <> quality) PSU would be a good idea. On a budget, the Antec VP-450 or 380W Earthwatts are good, reliable choices that can easily handle a HD7770.