This might be a problem, any cases that I have seen with smiler dimensions are HTPC cases that only support low profile parts and half height cards. Maybe consider the Evga Hadron, Its small and compact with a 500W power supply, perfect for any mITX build!
not really small, but does kinda seems more on the console/vcr/flat laying case style, is the mountain mods plateau 18. that's what i'm currently looking at for a setup of mine, only downside is that the price is a bit hefty
The fractal nodes are nice cases, and there small, square and in the 605s case wider than it is tall. Just like the new consoles and the steam box are. I think I will be using a node for my home sever when I get around to doing it.
[edit] However I should have mentioned the node 605 is larger case than you might be after (correct me if I am wrong), I would check out the node 304 though if I were you.
The main build i should be OK with. Either going to go with a Richland with a load of RAM or Piledrive the crap out of it and 280 or 290 (I'm a bit of an AMD fan, had issues with Intel and Nvidia a long time ago that put me off)
I already have one or two of the flexi's in a tub some where. Its looking like I'll have to get a full height case though for what i want style and power wise.
Try to get your hands on a old Alienware X51, they aren't much bigger than a Xbox 360, but they have a small PSU (330W). Other than that a HTPC would be your best bet.