Three comments:

1. What kind of conditions are you riding in that your current brakes are inadequate? You're going to sink some money into new wheels, or at least hubs, and it's very tempting to get 'better' wheels rather than equivalent. Discs have two distinct advantages over v-brakes: severe & sustained downhill braking, and muddy conditions.

2. Discs are heavier than v-brakes once you consider the rotor and caliper compared to just a pinch brake.

3. If you're still intent on discs after reading and (hopefully ) considering my first two points, then I fully agree that the Avid BB7 is the brake you want. The 160mm rotors are fine. You don't need the big rotors except for downhill racing; it's just extra weight.