When you say 'Internal' temperature, do you mean the ambient temperature in the case?
But, it really all depends on what the ambient temperature in your room is, you obviously can't get below ambient rooms temps, unless you're on some gnarly water cooling.
Also, if you're worried about making sure that your temps are Ok, It's the CPU Core and GPU Core temps that you want to be looking at, these are the ones that you want to keep as low as you can.
To monitor your CPU Core temp, download Core Temp;
Well, there isn't really any "Normal" ambient temp for the inside of the case, that temp doesn't really matter all that much, I'd be much more concerned about the CPU and GPU temps.