Well, I completely agree with what seablade just asked you. People usually use an operating system that fits their computer and their operational needs, not the other way around.
But, to answer your question, any recent laptop will be xgl compatible. Now, if you want to squeeze every last bit of performance out of that, you may have to use proprietary drivers. Once you have a laptop spec'ed out, check the ubuntu and gentoo forums to see what problems people are having with your particular GPU.
Lastly, the argument you are using demonstrates that you only need a very lightweight portable machine that you will only use sometimes, but that you are at your desk the majority of the time. Yes, it would be very hard to beat a $1000 desktop with a $1500 laptop. However, my primary (and only) computer is a laptop. So, it is worth it to me to get that laptop. Whether or not it is worth it for you is something only you can answer (or maybe even other people, if you provide more information on what you want to do on these systems you are considering). Do you even need one?