The case is a very important issue. If you have the room, going a bit oversize can be a very good thing. I have a couple older desktops where it is very hard to get to anything because everything is very squeezed. My Corsair 600T has lots of open space, routes most cables out of site UNDER the mobo, is amazingly easy to work in and stays nice and cool inside. \
A mobo consideration; as soon as you buy one, Intel will likely make it obsolete by coming up with its latest chips in a different form factor. If the CPU you are now buying is from an old line of CPU's, the mobo will quickly become obsolete; though that is even a risk for newer CPU lines.
PSU's are very important - not just the total official power output, but their quality. I have a 'gold' 800 or so, that really puts out its rated output power. But what's really nice is that when the system isn't doing much, it consumes far less power then you might expect (about 100 watts idling with 16gb, 2hd's, an AMD video card, and an I7 CPU). Also, my psu does not have most of the power output cables permanently attached, making the inside of the case far less crowded.
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