There's really no way to answer this because it all depends upon your girlfriend. If she is the type who likes the romance and wants the show and pomp of a royal wedding, you better make it good ;)
If, on the other hand, she is someone who is very down to earth and doesn't like a lot of attention and "hooha" then just keep it simple. Personally, I would be really ticked if my boyfriend did what seems to be all the rage right now and propose to me in front of a zillion people - I would be very ticked AND embarassed!
Having said all of that, whatever you do, you better make it sincere and not seem like an afterthought - although some women may not even care about that!