A temp of 48°C is very good. Anything 50-60°C for 3D gaming is common. Many people get 80-90°C for extended 3D gaming. FX 5800 operating temperature max is 140°C.
Under water cooling, my 5800 non-U runs 3D games at 26°C even when OC to 500/1000. I don't think I've seen anyone with a cooler 3D temperature.