How many HD's will my MoBo support?
 

It's and Intel Pentium III, 601 MHz with 320 MB of RAM. I have a couple old harddrive I want to slap into the computer, but I'm not sure how many it will support. How do I find out?

Just look and see how many connectors you have. If you trace back the connectors from your current hard drive, that should help you locate them. In case you're not familiar with the types, IDE connectors take a ribbon cable about 2" wide while SATA take a narrow cable about 0.5" wide.

IDE = 2 per connector on your motherboard.

SATA 1 per connector on your motherboard.

The most IDE drives I've ever seen in one computer is 4 whereas I've seen six or more SATA connectors on one system.

You always have the option of installing additional PCI/PCIe PATA/SATA adapter on your board. Assuming that the board already has a (PATA) controller with 2 channels and you have N free PCI/PCIe slots on the board, you can have:
(N + 1) * 2 * 2 harddrives in your system (two per channel: master and slave)

I'm assuming that the additional controller supplies two channels.

However I think your problem won't be IDE channels but space to install the actual harddrives :D

In that case there would be nothing stopping him from transplanting the guts of the PC into a case with lots of bays. I've seen cases designed for RAID fileservers, with 8 drive bays.

With only 2 additional PCI controllers you can drive up to 12 drives, I don't think you want to pack 12 drives, each 3.5 inch into one case. Drives produce heat as well, I really don't recommend running more than 4 drives in a case... without additional cooling measures and noise blocking.

I have currently two 750GB drives installed in noiseblocker boxes, fused together to a (in fact multiple) RAID1 device. Drives are SATA however.
Works quite well. Disk access is almost non-audible. Howerver I had to do some power management and AAM tuning to get noise that low.

You'd need some serious cooling unless all of your drives are solid state!