You don't mention your budget, so I am going to present a cheaper alternative to live rock. You can definately run a fish only setup successfully the old fashioned way.
On a 120 gallon tank you could possibly use 2x Penguin Emporer filters as a biological filter to compliment a hang-on skimmer. Order from thatpetplace.com and you can probably filter for under $200. SeaClone makes a nice venturi model skimmer for a tank of your size.
This is not the optimal setup, but with a monthly 30 gallon water change you would have a very healthy system. Just be sure to test Nitrates prior to every water change and make sure they are not rising above 40ppm. If this occurs, you will want to change larger volums of water, up to 50%. The long term cost of salt could get expensive, which is why live rock is best if you can afford the initial expense.
"Looking in windows, knocking on doors, they need to take seven and they might take yours."