I'm in the same boat. I currently have a 2007 Mac Book pro but when ever I try and load Linux distros (Ubuntu, Arch Linux) on it they do not work with my video card. I'm actually thinking about getting a sager or a eurocom laptop as well, but I'm not sure if they will work flawlessly with Linux, primarily Back Track 5R3 and Arch Linux? Honestly Mac's are nice, I love mine but they need so many work a rounds and hassle to run certain distro's that it is turning me off, especially since I would like to learn the command line more for my  desired profession as a pen tester. So if I were you I would looking into one of those laptops, plus I know you can get a Killer wifi nic that is Atheros chipset, that may work with out the need of a ext. Alfa nic. Good Luck! and if anyone knows about those laptops being great for Linux, please let me know.

Yeah I currently use a Alfa AWUS036H it works great. Here's a website that I purchased mine from <http://hakshop.myshopify.com/> when I got mine they had a cd compilation that came with the adapter for wifi hacks and all that.