Don't get a touchscreen for desktop use, it's a gimmick. If you want to do it properly, get a Cintiq.
Instead go get yourself a good quality 24" display, like the Apple LED Cinema Display.
I have an HP TouchSmart 22" at home. It is most definitely NOT a gimmick. I use it every day. The machine has the included wireless keyboard in front of it, and I stash the mouse away since it's not needed.
Now, I don't do work on this machine. I do light web browsing, e-mail, IM, some media playback, and that's about it. It's on a high table by the window near my kitchen, so I'm generally standing while I use it. I read news on it every morning and I love being able to walk up to it, tap on some links, pan through the pages, etc. It runs in high DPI so the text is readable from a bit of distance too. Working with a set up like that with the mouse is pretty uncomfortable and cramps my wrist.
For a workstation / typical desk set up, it may not be very useful. But I just moved to a new (small-ish) apartment, and so I got rid of my desktop PC which I realized I was never using anymore. When I work it's either at my office or on my laptop, and I don't really play games these days, so the TouchSmart made perfect sense and I really like how it fits into my lifestyle.
Oh yeah, I got mine refurbished for $900 (looks and works as brand new).