well, usually i'd say get the set.....BUUUUT, why get a lil more than half a carat for the same price as a whole carat? and it "appears" that when the big day comes, you could just buy a slim solid white gold band for like $100(given you'll be able to save more money), that would slip nicely underneath.....the engagement ring shows character to me, the band the commitment.....
another option(and i hate the idea if you do lol), is to put the ring you have on ebay.....