Thanks for the replies everyone!
So on ebay I found an Alienware m17x with a Q9000 and GTX280M-SLI, and an XPS 15 with a 540M, both for around $800.
I know the 280M is old-school, but with SLI, the benchmarks look pretty good, benchmarks on Notebookcheck place it above the GTX 560M. Outside of tesselation, is there anything I would be missing in modern games?
My desktop is a Radeon 6950 with all the eye candy on, and games look nice, but would it really be noticeable with the older cards, such as the m17x above?