the tek fixtures are nice. I just bought a 4 light fixture over at reefgeek dot com for a really good price. They also include any bulbs you want with the purchase. the 48 inch fixture is exactly 12 inches wide.
I'm running the fixture on my 120 gallon tank which is 24 inch in width and 24 inches deep. I've just set it up but from what I can tell the spread is quite nice with the fixture about 12" from the surface. Very even lighting from front to back.
for a 75 gallon I wouldn't suggest the 4 bulb unit. I think it's just too much light. You are going to be fighting the tank, have to use lot's of Co2 and do lots of trimming. Don't underestimate the power of these fixtures. They are quite bright. If you insist on using the 4 bulb unit I've heard of a guy that was using NO bulbs in the fixture with great results. As I understand it these fixtures will auto sense NO or HO bulbs. Pretty cool if that is true.
There's a great thread at the Planted tank Forum by Hoppy in the lighting section. He collected a bunch of data with a PAR meter on different types of bulbs/ configurations. He found you don't need as much light as you think. It also seems raising the fixture higher helps get more even lighting from top to bottom and lower intensity meaning less issues with algae, more stable plant growth etc...
http://www.plantedtank.net/forums/lighting/105774-par-vs-distance-t5-t12-pc.html