Think this depends a lot on how and where you work. I much prefer my laptop over a tower. At work I have a monitor hooked up to it so the laptop screen is just used for toolbars and windows. I can then take it home, on a plane or on the road to keep on working.
As far as computer size and speed, it all depends on what you're doing. Just got a new 64bit with 4gb of ram. Considering a couple years ago I was doing the same thing with about 128mb of ram I think I'm good to go.