Sapphire will soon have a non reference card for cheap that will feature a better cooling system than the reference HD Sapphire HD4850. That may be a good option for a low priced HD4850 once it is available.
(should be soon).
I you want a powerful HD4850 look at Toxic Sapphire HD4850, GeCube OC HD4850 and Palit OC HD4850. All offer better cooling and better OC potential than reference HD4850 designs.