George: A good guide to sizing speaker cable is:
http://www.pcavtech.com/techtalk/wire_size/index.htm
Based on that, I'd say 12 AWG is probably good enough for you, though it wouldn't hurt to go slightly larger if you wanted to play it safe.
By the way, in case somebody tries to sell you on it, biwiring will not reduce resistance. If you want lower resistance, thicker cables is the way to go.