So you'll be wanting a large hard drive at least 60gig if not an 80 or more? At least 512ram and a fairly high end processor and battery life needs to be good. Plus you'll be wanting a wireless net connection as well.
I'd look at a PentiumM based system. The pM's are designed for the mobile setting so have good battery life. The top of the range pM's compare to a top end pentium processor. Look into the graphics chip as well see if its got its own memory or shares system memory.
What sort of a budget do you have to work with?