That is going to depend on a couple of things. How blue do you want it? Do you want the whole tank illuminated or just areas? What kind of lights are you going to be looking at?
I started with the R2 Solutions Extreme LED Tubes(5 LED's per tube), I have three of them, hooked to a auto-dimming controller. I like these a lot. I also, have four sets of 2 LED's from Current(cheaper), they are spliced into the other set, so I can dim these too. They are not as bright the R2's. I will replace the Current brand with the R2's(3 more should do it) sometime down the road. Oh, I have a 4' 120 gal, just to give you an idea. It lights up the tank pretty nicely, I run them on full power most of the time.
Matt