I might go as high as 8 bulbs, and the tek fixture. Many here would say that you should go with the aqualux or icecap reflectors on a tank that tall, as they produce 20% more light at 18" or more because of better reflectors, but in this case, perhaps the tek is an advantage because the light drop-off would allow you to still keep LPS and softies lower in the tank w/o melting them.
The reason I say 8 bulbs is that then you can use bluer bulbs and still get great retults rather than running the GE6500Ks for all your output. Besides, you might need the 8 bulb...if not...simply dont run them as long each day...thats all. But having it there is better than to wish you did later.
My choice of bulbs would be 3x Geisemann blue+, 3xGeisemann aquablue (maybe one actually a GE6500K), and two actinic+. Shortly, UVlighting (formerly URI) is coming out with a highly anticipated actinic T5...so then I would wait for those rather than buying the Geisemann actinic+.