It comes down to do you want to build your own or to buy a pre-built machine? If you build your own, then go for any of the "quiet" cases with grommets for hard drive mounts, such as the Sonata you mentioned, or the Antec 3700 series (from memory I think SLK3700BQE), or the Antec P160 series aluminum case. Also, the Kingwin aluminum cases use rubber grommets and are 2mm thick aluminum.
The case is the right place to start for a quiet computer built from ground-up (meaning, not trying to make a noisy computer quieter). Next, choose your components wisely. Seagate or Samsung hard drives are quiet. Zalman 7000 series CPU HSF are quiet and good enough for the highest end CPU. For video, Artic Cooling Silencer is good for Radeon series, while Zalman HP80C is good for Nvidia series. Choose from a whole bunch of quiet power supplies, such as the A+GPB Titanium 420W or the Antec TruePower series (not silent, but fairly quiet), or any Zalman unit. Even many of the Fortron units are fairly quiet. Round out your system with quiet fans.