That PSU will have no trouble with a HD7750. Its 20A of total +12V current is probably not enough to reliably run a HD7770 for extended periods of intense gaming; close, but not quite.
On lowered settings, a HD7750 can play modern games quite well, especially at lower resolutions. At 1366x768 (720p), you'll probably play on high.
Its better than my 5450. And my 5450 is quite OK for Most Wanted. Then a 7750 will kick ass. And they dont mind playing a game on 800x600 because they are just kids remember? And they are gonna play on a TV and never seen any game eye candy. Not even a PS3. LOL So a 7750 is MORE than enough. And plus. Either way its gonna be a step up from the ordinary TV they are watching heh