Nope. That's a normal temp for HD4850, even that card can handle up to 100'C... yes, 48XX series cards are generates more heat, but still normal. For safety, just make sure your card is not above 80'C.
How much your budget? Why don't u go for 5XXX series? More efficient, less heat and dx11 ability...
I have a sapphire card with a non-stock cooler. It idles at 39 C (20% fan), and goes to 60 C (50% fan) at full load.
The single slot 4850's had trouble with overheating i believe.
Could you please let us know which card exactly you have?
If your in the market for a new GPU, why not go for a HD5750, it will perform about the same as a 4850, uses less heat, less power, and has DX11 support, There roughly the same price
HD5750 = £90 HD4850= £86
Also the 5750 has 1GB of memory = better performance at higher res. and with AA enabled.