It depends on how "heavy" you want the effect. The farther away that they are from the surface, the greater area they cover. I would buy at least two 460nm models to start and go from there.
i use 3 of the all blue bars. one powered w/ two slaves. they are 24" above the water and 7" out from the sides aimed in at 30*. i only use them when i want to be wow'd and for pics, as i was warned that they may be too intense for the lps i am keeping to use regularly.
what makes the powerbrites so awesome imo is their intensity. i have seen regular blue led bars like those and they just aren't the same. it's the 1-watt leds w/ optics that give the intense color pop.
I have 7 powerbrites on my 150g (48 x 24 x 30tall). You can kind of see them when the 250W MH's are on, but it's not overwhelming. I basically use them for dusk/dawn (1 hr each).
With only the PowerBrites on, it's amazing though!