Get another PSU. You don't want to burn out a fairly expensive Enermax 550. Get a smaller enermax and then another cheap PSU. You can find small PSUs in electronics catalogs (www.jameco.com,
www.mouser.com) that are not specificly made for computers, but are more general purpose. One of those might fit inside your mid tower.
As far as air cooling goes, mount the radiator somewhere near the bottom of your case, preferably sitting flat on the bottom, your case raised off the ground with castors, with an intake fan blowing air through the radiator into the case. Then have a "chimney" blowhole on the top of your case. This setup works best because it blows all the hot air straight up in the natural direction it takes. This also keeps hot air from blowing at your face from a front blowhole
