Buy a new tank. The tank is probably the cheapest part of this hobby. If you're going to make the jump to saltwater, get a larger tank.
I am using my 28 gallon tank from freshwater days, and I've already spent close to a thousand dollars on other stuff (live rock, live sand, protein skimmer, lighting, powerheads, etc.)--all of which is pretty much a necessity. Now the only thing holding me back is this darned small tank, which will probably get replaced in the next year or two (along with some of the other more expensive equipment, unfortunately).
Just as an example, my 28 gallon tank cost something like $100 or so. My light and protein skimmer--both necessities--cost about three times that combined.