T5, T8 etc is just the diameter of the bulbs, T5's being the newer, narrower bulbs like PC bulbs. Here is some info: http://www.lightsearch.com/lightnow/technology_T5.htm
There is a bit of different configurations possible... I use a Workhorse 7 ballast and 54W bulbs (from DIYReef.com) and reflectors (purchased at Specialty-Lights.com).
The bulb selection is sparse here in the US, I wasn't able to find regular 28W T5's. Hopefully they will catch on some day.