Tanya has great suggestions. Adding a bolero or very dressy (think beads, sequins, metallic thread) cardigan to a strapless or sleeveless gown gives you far greater versatility if a sleeved gown is not something you'd normally go for. The idea of adding a super-fancy top to your skirt could work well, too, and then you could wear the top with other items in your closet (black pants, dark wash jeans, sheath dress, pencil skirts, etc.) to make lots of other outfits.
If you want to buy a new gown and have any consignment stores or even thrift stores around, check them. I constantly see beautiful formalwear at thrift stores--often these dresses were probably bought as bridesmaids' gowns, worn once, and given away while still basically new. They would make great black-tie gowns if accessorized properly. For some reason I see more gowns with sleeves at thrift and consignment stores than in normal retail stores. Maybe because they're just not that popular/versatile and so become closet orphans and end up given away. Since most people don't wear formal clothes too often, buying consignment or thrifted formalwear will give you a very happy cost-per-wear ratio.
