that machine doesnt sound all that prehistoric to me,
firstly. kde is a memory beast, chuck it,
although mandrake is compiled for i586 rather than the standard 386 of most distros i never notice much difference unless im doing something like large selects in mysql etc.
if i were you, throw away mandrake and install debian 3.0 (woody), but you dont have to do that.
for your desktop, stick to gnome 1.4 (this flys along on my 400mhz laptop) and only install system services that you absolutly need (ssh, stmp etc..) for your browser look at galeon, it is alot faster to start than mozilla but still uses Gecko
also, my 400mhz laptop runs the gnome anjuta c/c++ IDE very nicely,
have fun
Ian
p.s.
boosalis, motif isnt a window manager, its a graphic toolkit like GTK, QT etc, it is also pretty old and fairly crap looking :-) (think netscape 4 era)