Might as well make sure it has a Core2Duo CPU in it and makes sure you have like 256 megs of graphic power to drive the screen. Vista can handle more ram but it is power hungry as well in terms of how much CPU and ram it uses and will suck the battery down faster. ie a battery that lasts 4 hours with XP will only last 3 hours with Vista.
You can turn off all the fancy graphics though to have it looking like Windows 98 kind of thing which will give you more power to run programs.
The "Vista eats batteries faster than XP" contention is not really accurate. With the same apps running on both OS'es there will be very little difference. Vista itself is smart enough to put most background processes (search indexing, defrag, etc) into idle mode when running on battery power. Some PC makers loaded early systems with a bunch of poorly chosen apps that did tend to use more background CPU resources. (Google, Norton, Vongo...) Most of them have improved at this point, and you can uninstall most of the "craplets" that come pre-installed on new systems to avoid most problems. Some of the latest Dell machines are pretty craplet free already.
Turning of the nice graphics in Vista will slow down snappiness the display, but it will free up memory. If you have 2GB or more memory this should be a non-issue. Contrary to lots of noise on the Net, the Aero Glass graphics in Vista have about 1% impact on battery power. (See http://weblog.infoworld.com/enterprisedesk...vista_aero.html or http://windowsvistablog.com/blogs/windowsv...tery-life.aspx)
I agree that a Core2 cpu, 2+ GB of Ram, and an ATI or Nvidia 256 MB video option is your best best. I've been really happy with the HP DV6000, and Lenovo X61 notebooks.
Dave