Well, there is good news and there is bad news. The good news is that a UDMA interface can be used with both UDMA hard drives and legacy EIDE hard drives, and will be a lot faster than an EIDE interface.
The bad news is that the stock kernels (as of 2.0.35) do not currently support UDMA very well. The development kernels (current is 2.1.114) do have some UDMA support, however, and there are a number of kernel patches available for currently existing kernels. In addition, certain UDMA interfaces that are add-in cards rather than built into the motherboard require either a patch or some trickery to use. That is why this document exists - to explain how to get the patches and work the trickery.