Get your self a good quality sound rated "a", thermal protected commercial ballast like Advance MKIII, or equal. Also consider the following: Electronic Ballasts and T8 lamps Like Osram QT2x32-120-lp, Magnetek B232-I-120-lp, or Advance 2x32(cant remember the rest) These ballast come with the ability to operate up to 4 lamps (ie-4x32, or 3x32 for 3 lamps, etc) in the code name and will light 4', 3' or 2' lamps (fo32,25,17).
Best application: You have a row of cove lights around a kitchen with single lamp fixtures, a few 4' a 3' and a 2'. If it was a residentual installation, chances are each fixture has its own ballast. Replace all the ballasts with 1 four lamp ballast wired to operated all the fixtures with new T8 lamps.
Bottom Line: you buy 1 ballast and 4 lamps vs 4 ballasts and 4 lamps. (of course this will work with 2 two lamp fixtures.
Other considerations: Instant start ballasts are easier on the lamps if you turn them on and off alot. Electronic ballasts operate at high frequences, so lest flickering and T8 lamps and ballasts produce equivelant light levels at lower fixture wattage so they are cheaper to operate.
Why change the fixture :? Chances are you will end up with a residentual grade ballast (not sound rated or protected and the only reason the average Home Depot style fixture is cheaper than the Commercail replacement ballast sitting besides it) that will give you problems with in a year. The fixture is the cheap part, the ballast is what drives the lamps.
Remember: wires must be enclosed in flex between fixtures.(ie no exposed wires)