A mid tower is fine. Just ensure that whatever you get will be easy to work in, and isn't too cramped. ie a micro ATX system is probably going to far in the other direction, unless space is a significant concern.
Based on your description, am I right in assuming that you want a fairly silent system? If so, I'd suggest replacing the stock CPU heatsink with an aftermarket product that has a larger fan. For moving a given volume of air, the larger the fan, the more quiet it is. However, this does impact on case size-the heatsink will take significantly more room. This is another reason to recommend a mid-tower over something smaller.
Mid-towers can also support more fans, and are therefore quieter-the fans can run much slower.
I have a large mid-tower with an i5, and both the CPU and graphics card are overclocked. It has 5 case fans, a CPU fan and a GPU fan. At idle speeds, you can't even tell the computer is running. It only ramps up when playing games.