They're cheaper, last longer, bigger screens, upgradeable, etc.. On the other hand, if you need the machine to be mobile, then you choose the other option you've suggested.
As to the requirements of the computer, you've given us no information about what types of things you'll be doing.. Think about operating system first.. Will Vista suite you?.... because most brand name computers are commonly found with Vista installed these days. (If you check for HP and Dell business computers, you may still find XP.) Or you can contact a custom shop in your area.. See what they offer.
Processor speed and amount of RAM is much dependent on the operating system you select.. 1 GB of RAM is sufficient for everyday uses on an XP machine but twice that much is best for Vista..
Hope this helps and let us know more.
Need some help, new electrical eng. student needs suggestions on which to buy. Laptop or notebook, which brand and if possible what requirements for each. Will have access to desktop so unit will not be only source.