ok for starters , a computer become obsolete, when your needs change!
(if you can see yourself useing the exact same software for 4 years the what ever you get today will be fine).
chances are however you wont.
(i know heaps of people still running old p200Mmx systems to surf the net, send email, use word etc and they must be close to 3 years old now.)
as for the matter of ram, win2k it is recommended you have at least 128Mbs of ram. That is your starting point. Look at the software you are running. Anything there need tmore than that? will you be running more than one application at a time?
256Mb would be roughly where you would end up. Remember on most laptops you can upgrade ram later so you can always add , or replace a stick.
hope this helps