A couple of things -
I've got an XPS, two years old, 2g ram Nvidia 256 graphics, Pentium M. I use it every day on a 160 meg file, and it works better than I would have expected. I know a lot of people that bash Dells, but this has been rock steady since the day I opened it up. I've got the full service warranty, and I've had to replace the display and the keyboard (both my fault!) and it didn't cost me a dime. Open GL doesn't work on this display. I've never had Revit crash, never had out of memory problems which our workstations with 4g memory have constantly - it just works. The battery on this with the 17" monitor (1920x1200 helps alot too if your vision is good) is about an hour, so I carry another one around with me. I'm usually plugged in except on the plane anyway.
My wife is an architect and has a new Toshiba Tecra Tablet (not sure of the specs, but top of the line and maxed out), and it handles Revit great for residential stuff (which she does). I find the screen way too small to use as a main machine - the open GL doesn't work there either, and with the tiny screen and lower resolution you spend all your time zooming and panning. The tablet is amazing for sketching over Revit (it's got a handy screen capture program built in), but Revit is so right-click oriented and scroll-wheel dependent that a simple stylus doesn't cut it. Add to that the fact that with the tablet closed down in a comfortable working position, the keyboard is gone. No more shortcuts, no more convenient text, no escape key...
That said, she uses it for Revit all the time, exactly the way everybody else does, without the tablet, and she loves it. Battery life is great (relatively, at least) and the ability to sketch over a screen-capture camera view is amazing (did I say that already?). If you could combine the best of all worlds, I'd probably pay anything they asked... fortunately nobody's done that yet.
hope that helps a little,
Jeff