Need lighting

You are going to have a problem with electrical outlets -- not having enough.
Having said that, you may want to have two moderate-sized skimmers instead of trying to find a single skimmer large enough for your tank. Check out the discussion threads under skimmers.

 

WOW!!! What an upgrade...it should look awesome if you take the time and do it right. Make sure you don't skimp on the equipment.
As for skimmers, I would go with an AquaC EV400 with a Mag Drive 24! It handles tanks from 120-600 gallons.
Lighting is going to depend on what you plan on keeping. Is it going to be a FOWLR or with inverts, or is it going to be a full blown reef tank? Once you establish that, then you can focus on lighting requirements required by the inhabitants. If I were you, I would break my tank down into sections. Then I would have (2) 400watt metal halides covering most of my tank and then have (1)250watt metal halide in the last area of my tank, say to light the right corner of my tank. The reason for this is not all corals require the same light intensity. Therefore, the area of my tank covered by the 250 watt halide would be an area where I would keep soft corals, gorgonians, lps, and some stonies. The other area of my tank covered by my 400watt halides would have Montipora and various Acropora that require more intense lighting.
You will have some heat generated by the halide bulbs, however, it may not be necessary to buy a chiller. If you keep a fan pointed at the top of your tank to allow for evaporational cooling, it should suffice in keeping the temperature of your tank water under control.
Best of luck with the switch in tanks. Take some pictures of the tank when it is done...

 

Antonio,
So how's the giant reef tank coming along?