How powerful of a hammer drill do I need to make holes in a concrete ceiling?
You can drill through concrete even without a hammer drill, really it's just a function of time and energy.  How many holes do you have to drill? How big and how deep? For two small holes, I wouldn't really worry about it (so what if they take 5min each?), but if I had to drill 20 large holes, I'd want something like a rotary drill.

Good bits will make quick work of the job too, and low end bits in the best drill will be slow and tedious.

For small holes (say 1/2" or less diameter, regardless of depth), I'd say use a standard hammer drill.  Anything larger, rent or buy a rotary drill.  For really big holes (several inches in diameter), you need a core drill.