I've got to agree with KcMopar. Things can go wrong very quickly in a saltwater tank, and it can be extremely frustrating. While the larger the tank, the more stable it is, it's also a lot more expensive if you have a tank crash.
I'd suggest you don't get a standard 55g tank. They're narrow, hard to aquascape, and have a smaller surface area. The tank itself is one of the minor expenses when compared to lighting, skimmers, live rock and livestock, so it's worth spending a little extra to get a better surface area and a built-in sump, etc.
Definitely spend time planning your upgrade. Look into equipment and stocking plans. Figure out which fish you like best, and build your stocking plan around them.
Try your 20g for a few months and see how it works out.