Raf's advice is good. Read through everyone's experiences and try to decide which way to go. Not to influence you, or anything, but if it were me setting up ODNO for a 55g tank right now, here's the setup I'd use.
I know you can use the Advance ballasts from Home Depot, but the Fulham Workhorse is far more flexible and has a higher power factor (=more light). It's the most popular ballast for powering 55w Power Compact bulbs.
I'd buy two, and then use them each to drive a pair of 48" tubes 2x. (Like the diagram Raf posted above
Then I'd use two different bulbs. One pair of Sylvania Gro-Lux (regular, not wide spectrum) F40T12 bulbs. They usually run about $5 a bulb at Lowe's.
For the other pair, I'd find a pair of F32T8 daylight bulbs. I saw some at Home Depot the other day on clearance for $5 a pair. They were 6500K so they should provide a brilliant white light that will balance well with the pinkish color-enhancing hues of the Gro-Lux.
You should end up with about 3.9wpg with this setup, and have an incredible amount of flexibility for future upgrades/changes.
I calculated the estimates wpg as such:
50% increase (2xODNO) on 40w tube -> ~60w, times 2 bulbs = 120w
50% inceease (2xODNO) on 32w tube -> ~48w, times 2 bulbs = 96w
Note: This is a very general estimate since most ODNO users have actually found overdriven T8 bulbs to be brighter than T12 bulbs.
Congrats on your successful setup, Raf!