I'm currently looking at Dell Latitude e5400 or one of the ThinkPads. However, please correct me if I'm wrong, and I hope I am, Intel WiFi 5100 and 5300 cards do not seem to be supported by BackTrack and the Graphics Chip cannot be fully utilized (i.e. achieve it's natural resolution).
You can get the 5100 to work just fine if you have a look around, it seems to work pretty well in my HP DV3505 (£500), infact I'm pretty impressed with it.

I do recomend you look at ubuntu and learn how to setup and use the tools included in backtrack, it'll be far more beneficial to you, it was for me.