I know for alienware laptops you can upgrade the mobile cards pretty easily. They have a nice layout, and you just call up alienware i think and say "I want 2 8800's in sli now" and as long as you have the newer laptops they just ship you out the new ones for an outrageous price i bet, and then instruct you to put them in.
But really, why do you want that much power in your laptop? you might be able to save money by purchasing a cheaper laptop with an 8600m (for around 700-900), and then getting a desktop and OC'ing it to a better level then your alienware laptop. This laptop your talking about with all the upgrades will probably cost around $4,000? Maybe more?
get a $1,500 desktop, and a $900 laptop and you have yourself alot more money in the bank and the same gaming experience.
Oh, and my friend has experience with alienware support... they are extreme nubs most of the time and they try to cheat you out of your money. They have the bios locked on his so that even though he has a 64 -bit OS, the laptop only recognizes half his ram (he complained so much they unlocked it, which shows how bad they are). They also tried to screw him out of the promised upgradability to the 8 series.
I would just get a cheap acer laptop with an 8600m and a desktop with a nice 24" monitor
.