I would prefer to at least use 14ga for any home theater wiring, and for a run of 20 feet (which will probably require closer to 25 feet when you factor in turns, offsets, and a little slack at the ends to facilitate making connections) I'd start to consider 12ga. When I did my surround speaker wiring back in January, I used an assortment of Radio Shack plates ( four speaker , two speaker , and one speaker variations) that were particularly useful for installing in existing paneling and sheetrock. Since you'll be working from scratch, I'd put a single gang junction box in at each speaker location and a single gang (or double gang if you'll have wiring for four surrounds) j-box behind the equipment rack. With a true j-box to work with, something like the Leviton Quick-Ports could also work well.

I've used Blue Jeans Cables' interconnects a bit lately and been pretty satisfied with their products - of the two listed, I'd go with the Belden 5000UE at $0.39/foot, but I'd also ask them if the stuff is in-wall rated as I didn't see any mention about that on their info page. Definitely check Parts Express before you order, though, as they may also have some good prices on similar wire.