Depends on what you want to keep. If you want corals that or completly photosenthetic (sp?) you will need (want) 5-6 watts per gallon. I am in the same dilema as you, I have a 29 gallon tank yes I know small, that has 130 watts of light. Its enough to hold simple corals but the big stuff I want, not a chance. Your not going to find a hood thats 30 inches long.......trust me........ive looked for about 2 months.Not pre made at least. The web site www.glasscages.com
has hoods that will fit that or they will make it............and they are really good quality. 56 gal tank..... thats 30 inches long? That sounds interesting as well. ebay has the generic brand........which I am still unsure about. its 305 watts in a 30" fixture. Sounds great........but I cant vouch because I dont know.