From the site:
Suggestions: (1) You may want to consider purchasing 184-pin DIMMs along with your motherboard if your existing memory is not of this type. (2) Be sure that you are using a Socket 7-compatible processor with this motherboard. (3) If you want to take full advantage of the on-board SATA controller, you will want to consider a hard drive upgrade. Existing IDE hard drives will run on the ATA-133 bus. (4) For motherboard add-ons, check out the storage and components section of CNET's website.
So an IDE harddrive will work easily. You can install anything on any sort of Hard drive really. 20gig's is enough space easily, but not if you have a ton of music and games on there as well.