175w on a 55 should be plenty; supplement that with PC or VHO actinics and you will be set for most anything you want. If you get actinic supplements you could try 6500k rating bulbs. If you do 10,000k it might look "too" blue. Look around at other people's tanks and find a look you like and get their setup.
Edit: Yes, you should get 2 175w bulbs. MH bulbs only cover a 24'' area and you want the full tank lighted.