We have had a 23" HP Touchsmart 600 for just over 2 years. We love it. Very quiet, although it does have a fan. Basically these computers are similar to laptops in design, so that should be an indicator of loudness if you want a comparison. Like a laptop, you won't really be able take it apart and work on the insides, but if that's okay, I'd say they are a good option. We love ours.
We also use a wireless printer to minimize clutter. Plus the computer is wireless eliminating the Cat5 cable.