There is space for two internal drives (plus the optical drive). HOWEVER, if you do not order the system with two drives, you will not receive the bracket and screws for mounting a second drive - make sure you buy those when you purchase the hard drive (the bracket has to come from Dell).
You can replace your optical drive with a hard drive by getting a caddy (ebay) for it. I have such a beasty running in many of my systems. They run fine and fast!!!
I got my 7720 at an unnanamed big box store and it had the frame for the hdd and the connections already there.
OPEN the bottom panel before you buy and LOOK to see if the caddy and connector are there, would be my advice.
A small, tiny, phillips head is all you need. Don't be shy, it won't break. Start prying at the right rubber foot (speaker side).