before you go all gung-ho for a laptop i'd encourage you to have a look at Small Form Factor systems.

I have been building them for years, and they really are pretty portable.

examples include:

Shuttle SFF PCs:
http://uk.shuttle.com/product_type.jsp?PCI=19
about the size of a shoe-box and available with a travel bag little bigger.
I bought a SN25P about five years ago, and its still playing L4D2 to this day.

Silverstone Sugo cases:
http://www.silverstonetek.com/produc...sure.php?area=
about the size of two shoe-boxes and available with a travel bag little bigger.
I have both a Sugo SG01e and an Sugo 03, the latter of which has 9800GX2 graphics cards and quad-core CPU in it!

the point being; you can still achieve portable without a laptop, and you can do so whilst getting literally twice as much power for your money.