So work decided to get rid of a ton of old computers, and I grabbed the best of the lot. It's an OOOOOOOOOOOOLD Gateway 2000 P1 @ 166Mhz. I wanna install a distro on there and screw around with it - which would be the best?

Is KDE gonna be able to run on it or should I use one of the lighter weight window managers?

Go check out C-net (http://www.cnet.com) search for operating systems and you should find quite a few to choose from. At one time they also had several lesser known OSes reviewed.