One thing you want to do is tweak what services get run and what services don't. How you do this will depend on your distro, but the basic idea is to only have things run at boot which you are going to need. In other words, if you discover that you have an irc server running at boot because you were playing with packages, then you don't want that, &c.

KDE should be happy with 256 Mb of memory. Still, you could always try a window manager instead of a full-blown desktop environment. I use fluxbox on my laptop; extremely fast.

-- Panem et *burp* circenses