If you dont feel like reading the very good advice from the page Adam posted, you are almost always safe going with 12AWG. Blue Jeans Cable and Mono Price has good cable at a good price. I have also used the Dayton speaker wire from Parts Express and it is good.
I have been searching for some wire that looks good (and I like big stuff) for a couple of months now. I finally found what I was looking for and it was delivered yesterday. It is from Best Deals Cable and it is very well constructed with nice terminations.