First let me thank Pave_Spectre and Inka for the helpful responses they gave to my last post...it's folks like these that keep me coming back to this forum, even when I don't have a question it is still a good read.
Now...my ancient system board (Asus P5A) supports "Ultra DMA/33 IDE" devices.
Question: Will a new HD that is EIDE ATA 100 work on this MoBo?

I think it will work, but it will run a lot slower than it is designed for. If I were doing it, I would get a PCI IDE ATA100 card to use instead of the IDE port on the motherboard. I have one for my 3rd hard drive in the computer I am using right now and it was quite reasonable in price at Newegg.

It will work. And while it will work at less than its optimal transfer rate, it is debatable whether that slowdown will be noticeable in ordinary use.

I think it was one of the first 2 cards on this Newegg page, (http://www.newegg.com/app/ViewProduct.asp?submit=manufactory&catalog=11&manufactory=1119&DEPA=1&sortby=14&order=1) but it has been a while, so I am not sure....