Well, with two E5430's, 16GB ram and a 7300GT you're looking at 350 - 400W, depending on your OC. If you don't plan on upgrading the GFX card, you'll be just fine with a HX520.
Glad you didn't purchase the Skulltrail though, as it would eat around 100W more under load

Don't really know about a case, one I know having the right mountings that isn't too big is the Silverstone TJ07. But it's very expensive of course..

I'd look around for a Chieftech Miditower, they are solid, not very expensive and usually have the right mountings.