Mckeyes nailed it...
First, take an inventory of the components in each system. Take each one apart and jot down the following...
- motherboard model (should be written on the motherboard in bigger letters)
- memory type & speed (240pin DDR2, 184pin DDR)
- memory size (should be written on the side, a sticker probably)
- hard drive type (SATA or IDE. Sata has a small cable with a little leg on one side, IDE is wide and flat with a bunch of pins)
- hard drive capacity (20Gb, 40Gb, 120Gb, etc)
- Power Supply size (sticker on the side)
Find out which case is heaviest and best ventilated. Pick that as the base. Install a fan in the back sucking out, and one in the front pulling air in. The largest power supply in the mix is probably still pretty low, so power consumption shouldn't be a big concern. Use the biggest you have available. Now for the motherboard. Look for SATA ports. The board with the most SATA ports is probably your best board for a file server. If you have more/bigger IDE hard drives than you do SATA hard drives, you'll probably want to go with an IDE motherboard. Only 4 IDE devices can be installed on one system at a time tho... remember that. So, if you put a DVD-RW IDE drive in, the maximum number of IDE hard drives you can have is 3. Sata, well, it usually has one IDE port for the DVD-RW and several other SATA ports for hard drives. It can have over 8 on a newer board. Being some of yours are 6 years old, you'll be lucky to find one with 4.
Now you have your case, power supply, and hard drives. Next, memory. Take the motherboard model number and type it in google. Find the spec sheet to see what memory sizes & speed types it can handle. Ideally you would match the memory up with the FSB of the processor, but at this point I'd say that isn't as important. Just use whatever processor is in the best motherboard... and put the maximum amount of supported memory you can fit in the board. While you are on the spec site, download the manual for the motherboard... it'll help you hook up the front panel connectors for the on/off switches and such.
That should do it for simple build. You'll install an OS and setup a network from there...