The standard HD 4850 runs at a toasty 80-90°C as a result of having 800 unified shaders humming away inside the GPU. AMD uses a variable-speed fan that spins up to speed as the graphics load rises, a strategy which successfully maintains a constant temperature.
However, you can distinctly hear the fan working away during a gaming session. It's not loud enough to cause concern but the high GPU temperature seems wrong, despite this being a deliberate design decision on the part of AMD. The HD 4850 chip has a high power draw and despite its high efficiency there's plenty of heat to be dispersed. The £125 HD 4850 uses a single-slot design that radiates waste heat inside the PC case, or you can choose a £175 HD 4870 which is a big, dual-slot card that exhausts through a slotted bracket to the outside world.