My experience with T5 is too limited so I probably shouldnt say anymore.
This should answer all of your questions about how MH work (plus it saves me valuable typing time).
Part V: Everything You Need to Know About
Metal Halide Lamps and Ballasts:
http://www.reefkeeping.com/issues/2007-03/sj/index.php
What temperature does the room your tank is in get to during the summer? If its in a cool basement you may not need anything. If its in a hot apartment with no air conditioning you might have a big problem with heat.
A single MH of either wattage should be able to light a 2'x2' area well with a decent reflector.