from what I think most people would go with 1 MH bulb per 2 feet of tank, and the Watts depends and the hight of the tank and what you whant to keep. how deep is your tank?
if you want to keep clams I would go with 3x 250W 10-14K and 4 x 110 VHO super blues.
well that is what I going to do on my 240 but with 4 x250 W MH and 4x 110W act.
happy lighting
Deva