You'll read that changing case does little to make your PC quieter. You have to start from quiet components... but if you already have quiet components, the case does matter. At least in my experience.
I'm not a silent PC guru, but thanks to this site and forum I carefully cherry picked my cpu cooler, fans, HD, GPU, etc. I wanted a small case so I got a Silverstone GD05, coming from a standard noisy PC I was quite satisfied by the very low noise of my new rig but after a while I started to notice it.
I read that the case doesn't impact that much on the overall noise but I ordered an Antec P183 anyway (I had a P180 once and it was great). Well, of course they are 2 completely different cases, but noise is definitely reduced by a good amount. The PC went from being silent to being (almost) mute.
The GD05 was vibrating too much I think, if I put a hand on the top of the case I could feel it vibrating, while the P183 is steady like a rock. In a couple of weeks I'm sure I'll start noticing the noise of this new setup as well... you can always make it more silent