What gauge speaker wire/cable should I use?

Use 12 gauge if you can (AND DO NOT RUN IT NEAR POWER CABLES). In most peoples opinion there is a pretty big audio difference between cable sizes, especially 18 gauge! If you have to save go for 14 gauge. A really good way to save money is to buy generic brand cables, don't get tricked by bestbuy and there "monster cables" BS, any brand will do. -just DON'T FORGET to not run the cables near the power lines, doing so significantly raises the chances of buzz and fuzz. -Also it sounds like your doing Home Theater installation, if your planing on buying HDMI cables -do-not- DO NOT get them from bestbuy or similar store. bluejeanscable.com makes HDMI and other types of cables out of commercial grade metal and shielding and they cost 10X less that anywhere else. Basically they're the five star resteraunts of HDMI cables while everything else is a Mc. Donalds. I ran a 70ft bluejeans cable to a projector with no signal loss, if you tried that with Monster cables you'd only get static (HDMI is rated officially to a max of 15' blue jeans is guaranteed up to 125'). -also for speakers, a little known brand that hand crafts all there speakers (also better than consumer grade for less) aperionaudio.com

Not that much.... Only an ohm or two...but current carrying capability is another thing....just how loud do you plan on playing it constanty? 80W ? or 100 W? Constant! I still suggest the 16 ga, because it's only pennies per foot difference, not dollars per inch.... You know the old saying, penny-wise but pound foolish.... And if you are running it thru a wall, it's just not wise at all....