What gauge speaker wire do I need?
The most important aspect of speaker wire is resistance. There is a ton of junk-science and stupid audiophile marketing claims that will try to confuse you and make you spend absurd amounts of money, but don't be fooled.

Wikipedia actually has a good write-up on it:


  Resistance is by far the most important specification of speaker wire. Low-resistance speaker wire allows more of the amplifier's power to energize the loudspeaker's voice coil. The shorter the cable and the greater the conductor's cross-sectional area, the lower its resistance. Depending on the hearing ability of the listener, this resistance begins to have an audible effect when the resistance exceeds 5% of the speaker's impedance.


They also have a chart: for 35' at 8&ohm;, you're good with 18 or 16AWG. 



All you need is basic stranded cable. Don't pay extra for fancy brands, or "oxygen-free" or "hand woven by the dalai lama" or whatever (unless you have the money to burn and really way to, I guess) as there's really no difference -- you can use lamp cord, if you want.

In proper tests, the best audiophiles can't tell the difference between $500/ft "top of the line" speaker wire and coat hangers -- and if you can, James Randi will give you $1 million (sorry, a bit OT from your question, but people that believe in this nonsense really piss me off).



For in-wall applications, you should get wire rated for in-walls - which basically boils down to fumes that the wire gives off in a fire. There's a good article on Understanding In-wall Speaker, Video and Audio Cable Ratings on Audioholics. In short, in a 1-2 family dwelling, any cable with the following ratings is suitable for in-wall: CM, CMP, CMR, CMG, CL2*, CL3*.