how old is your old nova extreme? if you have an older model, they don't have individual reflectors and the upgrade will be amazing.
the newer ones do come with individual reflectors and put out a lot more light. if the case is the latter, then you are picking up 2 more lamps and that's the difference.
with either fixture, you should change out the bulbs. current's stock bulbs are not so good. check out ATI and giesemann lamps. they have much better PAR numbers and make the tank look better.