I live in AZ, so don't know much about basements. But aren't their exterior walls brick or concrete?

If interior wall, wood, it shouldn't bemuch f a problem. I usually error on the side of too much rather than too little.

In other words if it is recommended to be 18 gauge, for your distance, I would go with 16. ;)

Also, I would think about what you may want in the future, like network cabling, etc. 

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I have done lots of speaker wiring in homes and offices in my time. If you are going to run to multiple rooms, use a multiple wire cable. Not one at a time to the different speakers in the other rooms. It makes the install a lot easier. Then once in the room, you can separate the different wiring. Wire is identified by its gauge or thickness. The normal thickness range for speakers can run from gauge 16 to 12. The smaller the number, the thicker the copper conductor, and the better its capacity to pass the amplified audio signal. If you are running long lengths (in excess of approximately 200 feet) of speaker wires, do not settle for anything less than gauge 12. I like the CL-3 12 gauge wiring. This is the UL approved cabling used a lot for speaker runs.  Check it out HERE.  You can most likely find it local where you are at if you go to an electrical supply house. When running the wiring, Do not run the speaker wiring through any existing holes that have electrical cable going through them unless they are the shielded like with conduit or BX cabling. Try and keep the speaker wiring at least a good 6 to 8 inches from electrical wiring. There are a lot of good articles on the web that may be able to give you a better understanding of this subject.  It is not hard to do, and if done properly, can give you years and years of no hassle music. Good Luck

Not a problem! 

I'm thinking about doing a home theater in my basement as well. If you are going to run any cable underground or outdoors, make sure you look for direct burial cables. We've got a great assortment of custom installation cables available that are CL rated and bundled. Check out: http://www.monstercable.com/custom_in...

also check out On-Q products, they make reall simple and clean products for integrating systems, or at least having wall plates for speakers and A/V.