Mousemonkey :
I always put the case last on the list, that way you can get something to put the chosen parts in rather than try to find parts that fit in a particular space.
Almost everyone underestimates the importance of a case.
Cooling.
Space (for now and maybe later too?)
Silence
First you check out the charts over here at TH.
Than make an estimation checking out the benchmarks, prices and availability of the (compatible) parts.
1.
For businesses and people at home I just keep in mind, for now and years to come, I need a good, sturdy case (usually a mid-tower) that fits the mainboard I have in mind (usually E-ATX fitted since it holds ALL mb's) and all the drive-bays I'll need (usually just two BDR's and a 3.5" or 5.25" plug-in panel that came with the MB).
That way I can never go wrong selecting a case.
2.
The PSU I need is 80 Plus Gold at minimum.
3.
For the MB I need one that would be able to hold the CPU I have in mind, ALL drives (I have in mind), ALL cards (I have in mind), the RAM (I have in mind) and maybe a few other peripherals.
4.
Select a good low price/high performance CPU.
5.
Check out first, the fastest RAM for that MB and second, as much of that RAM as possible in that MB.
And than plug in the selected low price/high performance VGA.
Keep in mind that that's the base.
Ya can build up on that choosing your own peripherals.