Wire for 50A circuit?

I need to buy some wire for a 240V 50A circuit run through the walls. Not hooking it up myself, just need help with choosing it.

Am I correct that it requires 6AWG wire? If I want to include a neutral wire in the circuit as well, do I need 3-conductor or 4-conductor cable?

And does anybody have any particular recommendations as far as what I should get? I need about a 50' run, and I know this stuff won't be cheap... but I'd like to be as gentle on my wallet as possible right now

Don't take any of my advice as more than a grain of salt but If you want a neutral you would need 4-conductors (2 hot, 1 neutral, 1 earth).
I found this wire size calculator http://www.csgnetwork.com/wiresizecalc.html. 6AWG is about 14 mm^2 right? that is some huge cable! The calculator says for a 120V (each hot will be at 120V)/50A/25' (1/2 of 50' - don't know why they do that?) you need 10AWG. If you are getting a sparky to install it for you just ask him what size you need when he quotes the job (and do you get some major discount for some raeson? otherwise I would think a sparky could buy the cable and charge his 10-15% extra and it would still be cheaper than you buying it )
Ok Ignore what I said and go with Boerderij Glad that bumping this got a better answer.

You want 6-3 awg. It is three conductors and one ground. I would just go to a big box and have them cut you 50'. It should be around $2/foot.