Since you are looking for silence and don't intend to run an SLI setup, a Sonata 2 might be a good case for you. Its built-in power supply would limit an SLI system but is more than enough for one high performance card, its case and power supply fans are very quiet, and airflow is decent (though not super). Oh, and it looks good, which counts too.
It is a smallish case though, which means that a large motherboard is slightly tricky to install - You almost have to drop it in, as there's hardly any room for fingers to lower it down... might be a bit terrifying if you're a first-time builder.
The P5b is ok, but a few quick web searches seem to indicate that an MSI P965 Platinum offers better performance and nearly the same features in the same price range. Plus, I'd tend to pick MSI over Asus on general principles, but I guess that's a matter of taste, both are good.