I’ve used KDE on a Thinkpad T60, it’s about 17 years old, has 3GB of RAM, and a Core Duo. It ran surprisingly well. Replacing the HDD with a SSD can also make a noticeable difference, so you should consider that if you haven’t already. I also turned off a lot of the animations and effects for better performance.
They’ve already been mentioned, but The Dead Milkmen are exactly what you’re looking for.