You're worrying about buying a space heater. But ALL videocards are spaceheaters. It's just that the HD4850 dumps the heat in your case, whilst dualslot coolers dump it outside your case, in your room.
You have to think of it like this, they all generate heat, somewhere around 90-110w of heat. It doesn't matter how hot a videocard runs, because that's something the cooling takes care of, it's just that the HD4850 runs a little hot, because the fan on it runs rather LOW. If you download the new bios, and flash it, it will have very acceptable idle/load temps though. And if you have sufficient airflow in your case, the heat will be expelled just as easily.