I've still got a few low-end servers at work using BP6's and dual 366's at 550. If you are looking for nice, low cost Linux boxes, these work nicely because you can use PC100 Memory (at 550 the memory is running at 100), they have HTP 366 on board, so you can do IDE Raid. They really are nice little work horses.
The only downside is that I would only use them if you have old memory and drives laying around since you can get soooo much bang for the buck buying newer stuff.
Cheers,
Joe