I prefer KDE for the primary reason that the versions I've tried have always been faster and more stable than Gnome. At work I ran a PII-300 and at home I still run a K6-2-350 with 128 Meg ram.
It ain't fast, but it ain't slow either.
My workstation at work is now a 1.1 GHz machine, and both Gnome and KDE are suitably fast on that platform. It also has 512 Meg RAM, which helps.
Can you add memory to your machine? 128Meg is still pretty small for a Gnome / KDE workstation.