Looking for a new laptop- Macbook Pro?

I just put my bike up for sale and as soon as it sells, I'm thinking of getting a new laptop. I want something that can run games. Doesn't have to run Crysis on max settings, but Warcraft would be nice. I'm thinking about taking the Mac plunge, and a top of the line Macbook Pro runs for about $3k. I'm seeing them on Craigslist and the Bay for half that, second hand. Is this a good decision? Or should I look for an Alienware or XPS?

Edit: Is 17" too big to be considered portable?