Johnny-
aside from overloading the amp (which has its own solutions) there are two problems with using one receiver for the main room and the other rooms. First, if you have any speaker-level volume controls (like the wall-mounted ones that allow you to adjust volume in the room you are in) in any of the other rooms, you have to have a speaker volume control in the main room. Otherwise the main room will be running full blast while you can trim down the other rooms. If your main room is set up for surround, then every time you want to listen to surround there, you will have to make sure that the main room's volume control is turned up all the way. Otherwise your surround setup will be out of balance.
The other problem is that whenever you switch to surround, the left and right speakers do not just get the left and right audio. In dts, or Dolby Digital, or even ProLogic, most of the program's audio will come out of the center speaker. Then the speakers in the other rooms, which are connected to the left and right channels, will sound weird because they won't have most of the program's audio. Thus you will have a system that can either be surround in one room or stereo in all rooms, where you will always have to make manual volume adjustments when you go from one mode to the other.
For cheap, use two receivers and Y-connect the sources. You may be able to find an old used stereo receiver. The only possible problem there is that some old receivers, when not turned on, will make the OTHER amp connected to the y-connectors sound distorted. In that case, you always have to have both amps on.

Good luck!