One of the guys at aquatic express said I would be fine with a 150. I want to get a light that I dont have to question if I have enough light for a coral or if I ever dip into clam keeping. I was told 150w should be enough.
Knowing a little about your intentions helps a lot. I have to agree with Kathy. A 24" deep tank would do best with a 250w MH bulb. Remember, you're going to position the light about 9"-12" above the water. If you have a 3" sandbed, your light will now be 30"-33" above the sandbed. Any coral on the sand that requires light (clam as well) most likely won't be sufficient.
more light can't hurt, but it can sure heat the water faster. I agree with aquatic express on the 150watt.
be advised that there are many corals that can be kept with these lights. for those corals that need more light simply place them on a higher rock in the tank.