Specification
Although manufactures claim that there are minimum levels of PC that will run there systems, we actually use PC's and see a lot of slow but new PC's... (compliments of slick sales people). Make sure you know what some of the main terms mean, and which ones matter.
Processor - Portable PC power, lags behind that of the desktop PC, (normally to conserve battery life).Get minimum of a Pentium II 350 processor to run your copy of MS Office 97...or MS Office 2000

RAM - Realistically, 64 MB of RAM is the least you would want in a portable PC. Remember its cheaper to get the RAM on purchase, than come back for more.
Hard drive - Somehow we always manage to fill the hard drives of our PC's. Ours are all more than 4GB.

Battery - Our PC's feature dual battery packs which make the PC run for around 6 hours. If you get a second battery, it will never be fully charged when you need it, or will be in your other case. In reality, sacrifice just that bit more weight for more power life. Remember it gets worse as the PC gets older. Choose the latest Lithium Ion technology for longest life with lightest (?) weight.

Display - Although some say size doesn't matter, ....with computer screens they could be just a bit wrong!. Ideally a 14" TFT screen should be on your portable. This will probably be better than your old desktop PC, and offers a high resolution of greater than 800x600

Multimedia - just make sure sound is on your PC. Whilst the size of the speakers may not be sub-woofer, 32bit stereo should be standard.

CD-ROM - What can we say, but ours all have them (admittedly we have to take the second battery out), but don't get one less than 20x. You may also want to consider the new DVD players which offer better quality whist retaining the ability to process CD's. 

PCMCIA - the method of connecting items to portables is now the standard called PCMCIA. Essentially credit card (OK a bit thicker) sized items such as modems, network cards, drivers for some CD-ROMs and even hard disks.