2 6x54 t5ho will be plenty of light for plants over a 8 foot tank at 2 feet depth. i agree with ed about the 6500 light temp but would suggest putting in one or 2 3k bulbs to help warm the spectrum up a bit. i am currently using a 50/50 mix of 6500 and 3000 but i will probably change the mix in different fixtures to see what it does to the look and see if it affects how well things bloom. lots of good advice, and plenty of odd stuff too, on the pot growing boards. they are very motivated to get good growth and lots of blooms on their crop.
I can suggest you put also LED strip of warmer color (about 3000K), or 2-3 compact fluorescent lamps / energy saving lamps as evening lighting.
Also LED are interesting for simulating godrays. You have big enought tank for such additions.