I installed LXDE and made it my default desktop. Everything's normal, except in the menu there's an entry called "Other", and it's about 3 screens tall and contains about 130 applications, including ALL the GUI ones that are already in the other sub-menus, plus many command line apps.
The UI is a tad faster now, but still very noticable delays. I'm pretty sure by now that the hardware is to blame. Considering I bought it in 2006, maybe I ought to drop the notion that I can still get a decent experience on it, speedwise.
On the other hand, I am perfectly content with the speed and responsiveness of Lubuntu, as well as Win7 on my netbook(!). It has 1,2GHz pentium dual core U5400, whereas the one on my desktop is 3,8GHz AMD x64! The bottleneck might be the RAM, which is 2 GB of DDR1 on a cheap motherboard (doesn't support DDR2) in the old big brother, as opposed to 4GB of DDR3 in the netbook.
I came across this article
a while after installing LXDE. It looks like this guy is doing what I wanted to do,
but I gotta say it's a whole lot more action to go through than just 1. install new desktop 2. set it as default.
Maybe the menu editor will help me clean things up in the menu.