I have been using that kind of a setup for a couple of years:http://forum.vectorlinux.com/index.php?topic=4564.0
You need an adapter that supports DMA. The card appears as a regular IDE device. No drivers or hacks needed.
Boot times are not significantly improved over a hard drive. General performance is also similar.
My main reason for doing this was noise. At this point I'm considering switching to a Solid State Drive. Although still expensive, I understand that performance has increased a lot lately, and prices have come down some.