I've been to a top surgery benefit before. They had a coffee can set out for random donations, bought a keg of beer and charged around 5$ for a cup for the night, and set a random price on shots you could have at about 3$. People feel more comfortable donating when no one can see how much each person has put in, like into a coffee canister.
I think a potluck/dinner would go about the same way. If everyone brought a dish to pass, it would cut down on personal costs. The important thing I think is to invite everyone you know who cares about you or your girlfriend, even family. If they don't show up, you may still be surprised by a card in the mail.
