Well, if it is RR, then there are holes drilled somewhere for a sump. So unless you plan to cap those holes, I'd say you need one.
There are a lot of debates about the necessity of a refugium. Some people think that they remove nitrates from your system. Some people think they are detrirus traps. I happen to fall into the latter category. Each person should make their own choice.
Like Lissa mention, if your tank is RR (drilled) with holes on the bottom of your tank, you will need a sump. In addition you will need a protein skimmer and a pump for your return.
(1) Skimmer. You can read the skimmer thread for good information.
(2) Return pump.
(3) Lights - you'll want to choose PC, T5 or MH based on what corals you want to keep.
(4) Powerheads. The amount of flow you want will again depend on the corals you want to keep.
(5) Test kits. You will want Nitrite, Ammonia, pH and Nitrate at the very least to begin.
(6) Phosban reactors. It's always good to start off with GFO and carbon as part of your system.
(7) Live rock and sand. For a 90 gallon tank, you'll probably want a minimum 90lbs of rock.
(8) Refractometer or hydrometer for checking your salinity.
(9) Assorted nets for when you get fish.
(10) If you are going to have corals, you'll want some kind of calcium additive eventually - either 2part or Kalk.
(11) Salt.
(12) Buckets.