Maybe a bit overkill but have a look at the Wiki:
I have 2 networkcard on my pc... one is wireless and other one is lancard. I want to use the wireless networkcard capture the internet and use lancard share with other pc. this is very easy to do on windows xp. but now I want to use backtrack to do it, because I have a HP compaq(t5000) it does not have a harddrive. but I can boot backtrack with a usb stick. and it works fine. I was wondering is that possible internet sharing on backtrack? I really want use this pc as a internet server. if some one know how to do it, please let me know.