I doubt you'll have any problem with heat to be honest, as long as your CPU and/or GPU isn't overclocked. Even then as long as you have 2 half decent case fans running correctly you should be fine.
Personally I got a very cheap mATX case with only 2 case fans running (as the case I wanted was not in stock) and I have no heat problems at all. The case is usually 37c or so and the mATX case would be a lot smaller than your standard ATX case so I have less room for air flow and its all good.
Hope that helps