What size tank to up grade to?

Hi all, I have a quick question to throw out there at ya. I currently have running a standard 55 gal w/2x175 watt halides. The question is what would be a good/bigger tank to up grade to(no bigger than 90,thinking maybe 72 bow ?),but yet still allowing me to use my halide set up.from what i have read I cant use 175's on anything deeper than 20 ish inch's(can bulid new canopy if need be). Im not sure of the different sizes/shapes of tanks out there. Going to have more room in the next coming months and would like to research this out prior(keep in mind this would be on the second floor with its own room). Thanks in andvance for any input.

 

i want one of those 90 wave tanks, they are alot$$$$