I'm a little confused on whether or not your current fixture is a fluorescent fixture or an incandescent fixture, to begin with.
T5, T8, T12 are all straight fluorescent tube bulbs. T5s are the smallest in diameter, with T12s the largest. Shop light fixtures usually take T12 bulbs. Standard aquarium fluorescent fixtures usually take T8 bulbs. T5 bulbs are newer on the market. Since these bulbs are so much smaller in diameter they produce more usable light per watt than T8s, and I'm pretty sure that they won't fit in T8 fixtures, either (though I can't say I've ever tried to be sure...)
If you've got an incandescent fixture, then I'd recommend two U-shaped Compact Fluorescent bulbs, you can find those at www.lightbulbsdirect.com
for pretty cheap.
How much light do you want? What's the rest of your tank setup like? Size, plants, what are your goals...
Do you want to set up a low light/ low tech setup, or do you want a high light CO2-injected setup?
Might be a good idea to spend some time at www.RexGrigg.com
reading up on the differences between high and low light planted tanks.