There is more profit in brand name speaker wire and interconnects than in virtually any other type of AV product. Monster Cable is probably the easiest example.
Steer clear of brand names. If you have any runs approaching 35 feet, then 12 gauge is a 'never have to think about it again' choice.
Try your local mega-hardware store, i.e., Home Depot, Lowes, etc., and a spool will be the least expensive.
And, like Chris said, get enough. And I would add, and then add a little more. You never know when you might re-arrange your speakers, or move.
Just as an FYI. Some years ago, a scientist did a study comparing everything from solid copper battery jumper cable, to rather expensive 1.5 inch wide multistrand woven wire (more fit for the inside of a computer!), and a number of high end speaker wires. At the end of the day, the difference from worst to best amounted to about 0.25 db at 20KHz. This kind of difference is considered just barely audible.
Those are some awsome charts, certainly enough to make my head swim. What I failed to notice, however, were any FR measurements. I must be confused. With all those numbers, what are we to make of the effects on FR of those pages and pages of charts?
You'd need to know the output impedance of the amp at various frequencies as well as the impedance of the speakers at various frequencies which could then be used to model the degree of FR attenuation. Or....you could just say that you want a thick enough cable without an extraordinarily high capacitance and move on to more challenging matters such as dealing with the room.
Andrew, great articles on wiring. The one that catches my interest is the artice entitled "A Practical Realist" by Brian Florian. Basically, he states: He used in-wall Ultralink CL414 cable 14 gauge 4 condutor wire that he purchased it for 75 cents a foot. I am doing a quick check for in-wall wire and I am unable to come close to his price. What would be a fair price for this wire by-the-foot and where do you buy it? Thanks, Gary