Byron, it's a 24" tube :)
As far as the 2wpg rule, throw it out the window. It's a more inaccurate than the inch per gallon rule, which is pretty far of too.
The only real thing we can use is 'lumens' which is a measure of light output, not wattage.
Maybe one day bulb manufacturers will list PAR (photosynthetically active radiation) but right now, they don't.
For low tech, (natural) that bulb should be fine.
Back when planted aquariums and flourescent lighting first met, there was only one kind of flourescent bulb, available in one spectrum (basically 'cool white') It was not nearly as powerful as the flourescent tubes available now, so you had to use 2-3 watts per gallon for good growth.
As technology progressed, the flourescent bulbs became more effecient by leaps and bounds, and then they developed a slew of new types- T8, HO, VHO, and more spectrums and light blends than you can count.
For example, I use CFLs (spiral bulbs), so even the smallest one (10W) is on the average-high side for my 10 gallon, I have two just so the light is even. It's definately high-light, but since I don't fertilise, I added a bunch of floating plants to filter the light.
Also, asking the best substrate is almost like asking "What's the best brand of car?" or "What's the best color to paint my house?"
As was mentioned, many people here use different substrates. Personally, I use soil, but I'll definately admit it has it's drawbacks. It's just a matter of preference.
Do make sure you get flourish comprehensive for that tank.