14 gauge will do fine for 50' lengths. Radio Shack sells some that has a clear jacket <http://www.radioshack.com/product.a...name=CTLG_007_002_003_004&product_id=278-1273> that I wired my living room with. I use Belden in the studio...actually, come to think of it, I just replaced all that with active monitors, so I'm using mic cables instead! But you can go to a Home Depot or such and buy stranded copper cable of whatever size...the downside being that it doesn't look as professional unless you run it in spaghetti.
Bigger is better: think of speaker wire as the hose coming out of a fire hydrant. A garden hose (small diameter) will transfer water... but fire hoses (large diameter) are what the firemen use.
That said, there's probably a limit on how much you gain as you go up in size, but it never hurts to get the biggest that'll fit. RS also sells solderable tips to attach to the ends of oversize cable if your speaker junctions won't handle big wire. Which leads to another important point: make sure the connections are the best you can devise. You are connecting current to the speakers. No sense running 8-ga all over if the signal never makes it across the gap to the speaker. Look in Gary Davis and Ralph Jones' Sound Reinforcement Handbook for all the math.