http://www.product-reviews.net/2011/05/02/alienware-m18x-specs-most-powerful-18-inch-gaming-laptop/
but i mean this thing is ''the most powerful machine in the universe'' it can use your eyes to lock it, and open pro grammes just by winking or squinting its high tech stuff. Its estimated to last about 4-5 years before all its tech is on the average laptop.
if you decide to go with a gaming laptop (instead of desktop + laptop), then you might consider that Alienware, but also take a look at some of the Sagers and other gaming laptops on http://www.xoticpc.com as Bewareofme suggested above.
You want to make sure to get one with display resolution at least 1600x900 or 1920x1080, recent dual-core or quad-core CPU, at least 4-6GB RAM and with at a minimum 1GB NVIDIA 460m or 1GB AMD 5830m cards (or better).
I have an HP Envy 15 laptop with the 5830M, i7-820QM, 16GB RAM, dual SSDs in Raid0 and it delivers decent framerates for just about any recent game at 1920x1080, with DX11, high res textures, SSAO, 16 x AF, 2 x AA. I don't know if it will handle games like Metro 2033 or Crysis, however.
BTW 18-inch for a laptop is incredibly huge and not convenient to carry around with you everywhere. I'd suggest to check one out in person at a retail store before buying.