I've never actually seen an Antec Sonata in operation, but I imagine this would be very close to it in terms of being quiet. The PSU is very quiet, but I replaced it with a Coolmax Taurus (~$40 underrated PSU that is very highly rated over at silentpcreview.com forums). That seemed to drop the noise slightly. For my CPU, I've got a 92mm Panaflo (on an xp-90) on a fan controller that I can turn down to ~5v which practically makes it silent. Once I did all that, the stock 120mm fan on the back of the case became the loudest component.
Be careful too, because this case has an "air duct" on the side that allows the CPU HSF to pull in cool air from the outside of the case to help cool off the CPU. If you have a slightly noisy HSF, I imagine this could make it slightly more audible.
To sum it all up, when I was using this case with the Coolmax PSU, and the 5v panaflo fan, I couldn't hear my system running when I unplugged the 120mm fan. If you were using this for a HTPC solution, I think the stock PSU would be fine. I would, however, replace the 120mm fan with a low rpm model, or, just unplug it...the Antec PSU would be exhausting enough air. Also, try to get a passively cooled video card, or a quality card (like my leadtek 6600gt which has a pretty quiet fan). I'd also suggest using a quality thermalright heatsink, with a low speed panaflo fan to cool your cpu.
Hope this helped, if you've got anymore questions, just ask. For what its worth, I'm thinking about ordering this case again intead of a cheapie Rosewill for my brother's build.