My first sw tank was a 55, and about 4 years ago, I upgraded to an oceanic 58 that is more cube-shaped and lost a foot in length, but gained 6 inches front to back - and it made a HUGE difference in the look of it.
Sadly, the 58 broke, and i had to go back to a 55 temporarily - and I suffered! Chief is right - it is definitely do-able, and you can even do it really well. If you can afford to go bigger, you might want to consider it, but don't stop from doing it just b/c you have this tank and can't get another one. IMHO.
In my experience, tho, one of the drawbacks to a 55-gal is that w/o a sump to increase the volume of the water, it can be difficult to maintain steady state for some key parameters - including spec. grav and temp. So if you do go with a 55, try to get some more volume with a sump!