If you have to do heavy compiling, the processor is important - otherwise, the RAM is the factor.
A good thing to do with low-end systems is to examine how much you actually need X - the graphical part of things.
Documents can be edited and typeset with
LaTeX in Vim - you can listen to music with
Mp3Blaster.
Mutt is actually a brilliant mail client, and a lot of sites can be viewed with less crap with
Links.
I heard that MPlayer will stream from framebuffer without starting X.
I am not saying that you shouldn't be using X, I am just saying that I personally only start X roughly half the times I use the system. I do a lot of homepages in Vim.
