Sorry I took a while to notice this.
Anywho, a 24" tube would be fine for a 40 gal breeder. You have to (well, not have to, it's just a much better idea) mount it inside the tank (just the uvb) as the screen filters out half of the UVB the beardie needs. I can give you a list of the stuff I use in my viv.
I use a 150w basking light (
http://www.petmountain.com/product/...72/zoo-med-reptile-uva-basking-spot-lamp.html) on a dimmer to monitor temps.
I also use both the fixture and light I linked above (24")
I use ceramic tile as a substrate, easy to clean and holds heat at night. Beardies can see the night bulbs, so those are pretty much useless for em. If you need supplemental heat at night (the temps can drop down to 60-65 at night) you can use a Ceramic Heat Emitter. They screw into your usual light fixture like the night bulb would.
I use a bunch of driftwood for a basking spot. What are you using to monitor temps? Stick on thermometer or digital?
EDIT: The coiled lights are infamous for causing eye problems in beardies, and the reptisun will replace the coiled. As said, the night light is useless, sadly. Sorry D:
Sorry to make ya buy everything over, Petsmart people rarely know anything about the animals they sell, sadly.