1) I will probably want to dual-boot with something Unix-y (Ubuntu or OS X will do), and Win XP (probably not Vista). Need a decent amount of disk space - the more the merrier !
2) One use will indeed be games, under Windows. I don't really need the absolute maximum gaming performance, though, as I generally don't buy games just after they've come out - I often pick up things on special a couple of years down the track. I do want to be able to replay all my old games, though, all the way back to Civ2 and SMAC. I also want decent performance for things like Oblivion, and will be picking up Bioshock and Supreme Commander in the near future. WINE gaming or VMware gaming don't really interest me - WINE because I've never had much luck getting games to actually run (even ones with a "gold" rating in the DB, e.g. BG II), and VMware because some of the games probably do need the computer's full attention on them to run properly.
3) I'm not actually sure how important size/portability are to me. Not even sure if I want 15" or 17" (or something else). I imagine at some point I'll have an external monitor hooked up to it, so 15" may not be a huge drawback ... on the other hand, 17" would be nicer when on the move.
4) I'm also not quite sure what my budget limit is. I have few doubts that a MacBook Pro would be a very nice machine for me, but configured the way I want it it would be nudging £2k, which is indeed rather expensive. I keep thinking that a non-Mac laptop suited to my purposes should be available for somewhat less than that, even if it isn't quite so Shiny!™.
For reference, my current desktop machine has an Athlon 2400, 2Gb RAM, GeForce 5900XT, dual boots XP and Ubuntu 7.04. Doesn't really run Oblivion properly - I had to run it at 800x600, and turn down/off most of the effects to get a half-decent frame rate (15-20, except in fights where it dropped below 10).
 Ironically, SMAC plays ok under XP (with the XP patch), but I couldn't get the Linux version working under Ubuntu 7.04.