[Myka]

Imo, it depends whether you're more into fish or more into corals. If you're really into corals like I am, then working with a smaller space isn't an issue. You can always sell a few of your less liked corals to make room for new ones. That I was did with my 33g. If you're more into fish I would suggest you get a 4 or 6' tank instead of going with a cube since even though cubes may have a significant number of gallons, it doesn't have the same amount or length of swimming space that fish like Tangs need.

As far as your particular tank dimensions go, I prefer taller tanks as to shorter tanks as I find them easier to aquascape. I find shallow tanks have little upper swimming room for the fish once the rock is in there and you add some corals, not to mention when the corals really start to grow! I agree that you should go as big as your budget and space allow, but to stick to efficiency as well. By that I mean, decide on lighting as well, and you can plan the tank as a whole instead of just around one aspect.

For a cube using one light, the maximum efficient size would be 30x30x30, but that's quite a big tank. This is efficient because a single halide with a "special" reflector designed to spread the light (like a Lumenbright, Lumenmax, or Lumenarc) will spread the light to a 30x30" patch. It is also easy to light 30" depth with a halide light.

My personal preference for a cube would be 24x24x26", but I'm not a huge fan of cubes just because the aquascape is so limited. I like to use halides supplemented with T5s, so the 24" will accomodate that.

I would reccommend against the centre overflow since it really cuts out the space for aquascaping, and really distract from the look. I find they look more inconspicuous when they are in a corner, especially when the overflow is black and you paint the background black.

Really it comes down to figuring out what you like, and what you want out of it.