Lighting for a reef tank is probably the most important (an expensive) thing you will have to buy. T5 HO bulbs come in a range of sizes and prices and you should be able to find bulbs that fit your fixture. Make sure the new bulb wattage is not too high for your old fixture.
It sounds like you may be having problems with your fixture anyway since you said one was brighter than the other. This can be a sign of power issues, if the bulbs are the same age. FLs should be changed at least every 10 months because the chemicals inside them do wear out over time. Old bulbs will actually produce a different kind of algae (which is not even real algae--it's bacteria) in your tank that you DON'T WANT. And ALWAYS recycle your bulbs intact! Most people just break them and put them in the trash. This is the worst thing you can do, believe me. You do not want to be exposed to the lead, mercury, and other supremely toxic chemicals that are in them. They are poisonous to you and persist in the environment for hundreds of years if released. There are recycling bins for these at WalMart, Home Depot, Lowes, and maybe even your LFS.
If your bulbs are the same age then you probably HAVE to get a new fixture anyway b/c this one is going to die on you. I would hate for you to buy brand new T5 HO bulbs and then lose your fixture. Start saving up now and by the time you're ready to go reef, you have the money to buy a good reef fixture. If you end up having to buy a new fixture you can always put the old fixture to use in a quaranteen, sump, or refugium for as long as it will last.
Someone said something about metal halides but I disagree. I'm not a huge fan of halides because they are so hot! Just because they are cheaper to power does not mean they are more efficient. That is a common misconception about halides. They are ridiculously expensive to buy in the first place and though they may use less energy, they waste it by giving off so much of it in the form of heat. They may even super heat your tank if it is not tall or deep enough and you don't suspend them pretty far over your tank to keep that heat energy from transferring to your tank.
T5 HO lights are more energy efficient because they do not waste the energy that they use; you can practically put them on your aquarium and if it has a good internal fan, it will not raise the temp in your tank my more than a few degrees. If you are only planning to have easy corals anyway, halides would be like bringing a REALLY EMPENSIVE gun to a pillow fight. Good Luck!