Picked these up yesterday. The black tower was an ebay auction that happened to be local to me. I won the auction for .99 cents US and when I went to pick it up he asked if I wanted the 17in MAG LCD. It also game with a X1650 Radeon. He said he was having trouble getting the Video card to work with the system but that it works fine with the onboard video and the video card tests fine in another machine but didn't want to mess with it anymore.
The ASRock motherboard is a 970 Extreme 4 which is technically modern as it's an AM3+ socket but I got it for free from another guy who said the only thing wrong with it is the onboard sound does not work.
The other three towers I got for 5 USD each from someone who got them in the house he just moved into.
The one which doesn't have a complete case is a Gateway 2000 Pentium 200. It's an ATX motherboard so probably worth transplanting into another case. The Compaq Desktop is a fully working Pentium 3 733mhz. The last tower is a whitebox Celeron 300 with an 440LX chipset motherboard. It won't post but hopefully It's something simple.
The Gateway Pentium machine and the whitebox P2 seem to have some good ISA sound cards in them. an Ensoniq Vivo-90DB-A and one I can't identify with a Yamaha YMF719E chipset.
All in all I spent 16 USB plus the gas to run around I think it was worth it. Here's hoping I can get the Celeron to post!
EDIT: The Celeron posts!, Just had to reseat the memory.