I'm sorry, and please don't mistake this for trolling, but I think that is a bit of an oxymoron.
Notebooks generally don't do games well enough to be considered for use by 99% of gamers. I think you'll find that a desktop, even a mATX one if you are concered about size, will offer a HUGE boost in your gaming. I looked into this myself a few years ago and was looking at $1600 books just to come close to 70% of what a desktop worth $1000 could do.
Also, as Dice144 mentioned, you can't just drop in a new GPU like you can in a desktop. Besides the interchangability issue there is also power draw to consider. Notebooks don't play well with high power requirments. Kind of kills the point, not to mention small-form-factor heat dissipation issues. Then you have to consider if the rest of you system is capable of feeding a new GPU without running into a bottleneck.
Either A) I'd look into a new SB based book with the option to use the CPU's onboard GPU for low power situations, and a dedicated GPU (perhaps even an external one) for power use. Or, B) Look into a small form factor desktop. I think one of the Anandtech reviwers did this about a year back and built a bangin' mATX system. I'd look into his results and don't forget to factor in modern components.
"The clarkdale experiment" by Loyd Case
http://www.anandtech.com/show/2924
Feel free to post questions / feedback!
Cheers!
Facebook
Twitter