Well honestly for that tank I'd go halide. A single 250w halide would be a great upgrade, and if you buy a pendant or DIY it, not too expensive.
I recommend halide only because with the depth of the tank being 24", you want good penetration to the bottom. While I'm sure some of the T5 crowd will disagree with me, it will be difficult to stuff enough 24" T5 lamps over the tank to achieve decent penetration of 24".
With that setup, you should be able to keep anything you want. If you wanted to be sure that high light stuff like clams, anemones, and SPS would do well, you could go to 400w, but that's approaching overkill.