What sort of games are you interested in?
If you're looking towards modern games, say from the last 18 months, the graphics card in your portable rig will be important too, as games from the last 6 years or so will do well with he right graphics processing. What do you have on your laptop?
If you're playing a game that demands keyboard-mouse heavy usage, such as a shooter game, you'd do well to invest in a good USB mouse and keyboard to avoid fatigue on your laptop, and to play the games on a separate table and chair to avoid strain on your back and wrists. You'd do well to make good use of a mouse anyway, as strategy games use them a lot.
Of course, your laptop should play older games like the original Unreal Tournament, Quake III, Deus Ex, some of the older Total War titles, etc. with considerable ease (Vista notwithstanding) as well as less hardware-demanding recent titles such as Peggle.
Personally speaking, you can't go wrong with adventure games either, nor with much older RPG titles. Yes, I sound like a broken record player but I'd recommend
Planescape: Torment (if you can find a copy) to any aspiring RPG/adventure gamer with a laptop.
