Well, the motherboard is going to run you about $80-$120, depending. (mITX mobos are kinda expensive.)
And then the i5-3570k is $215. (And you shouldn't even consider buying an i7. The only difference between it and the i5 is that the i7 has hyperthreading, which games don't use anyways.)
I actually highly recommend the Bitfenix Prodigy with a new i5. Newer processors and graphics cards are extremely efficient, so there's absolutely no worry about heat, and the Prodigy is a great case. I side-graded into it last year for the same reason you're considering, and I haven't had one issue between taking it home, to LAN parties... or on the airplane as a carry-on.