If you're running 40' I would definately use 12 gauge. The thicker wire (12g) will have much less attenuation (signal loss) than the 16 gauge. The longer the run you have the thicker the wire you want.
It's not as long a run as I thought. I gave it quite a bit of extra when playing with placement, and then when I found the spot I liked (back corner by the listening positions...big shock), I cut off the excess. It's still about a 30-foot run, though, so I think I'll go get some 12G this weekend.
On a side note, I EQ'd the hell out of my setup, and I'd hate to go through that crap again. I'm hoping it doesn't change anything that way. Thanks again.