Let's get a little specific here, just my opinion so listen to others and do your own research too..
Motherboard should be socket 1155, that way you can start with an extremely capable quad core such as my i5 760 2.8ghz BUT you can slot in an i7 somewhere down the track.
Make sure it has 4 ram slots for maximum memory upgradeablility.
USB 3 is cool, (charge USB3 devices), as is provision for external sata 3 connectivity.
PCIe 2.1 spec is most desirable for a graphics card which should have a minimum of 1gb of ram onboard. I don't buy into the half a dozen graphics cards scenario, (xfire/SLI).
I usually use power supplies of 700 watts for this type of system.
The case, (mid tower in your situation it would seem), must have adequate airflow. You may even consider modifying your PC bench to ensure proper airflow depending on the case. No one will see a hole you have cut in the side or rear for example.
Two sata optical drives is always a deluxe feature to have, spoil yourself
I built a system like this for a very good friend and his family, (wife & 2 teen daughters). They waited until they had the cash spare and we pounced and built it. Apart from them being amazed at its capabilities, everytime I go to visit, their system is buried in portable device usb cables and looks like an octopus. Once the girls found out it could charge mobile devices via USB3, lol, its seach for the deluxe tower itself lmaooo.
Poor mum and dad don't even get a look in as far as using this hotrod is concerned. If we had thought a bit more about it we would have built 2, one for the girls and one for mum and dad
