First of all, the clownfish. They actually do need at least 30 gallons. You may want to get only a simple, 30 gallon fish-only setup.
If so, you will need:
A tank
A stand
A heater
Some form of filtration (sump, canister filter, etc.)
A protein skimmer
A light (since it's fish-only, just about any light will do)
A cover
Salt
Substrate
Live rock
That's it. Whether this is vastly expensive or relatively cheap is up to you. If you get a big setup, with a sump, big lights, etc. the tank will be rather expensive. If you take the cheaper options, it will be less expensive. However, if your girlfriend decides later that she wants an anemone for her fish, or a coral, or some other form of invertebrate, it will be much easier to get it if you buy the more expensive setup.
This is not a light purchase. You will have to change about 20% of the tank's volume (6 gallons) every two weeks. You will need to make up your own salt water, and use it to replace the old 6 gallons. You cannot become lazy on this and start skipping it! Your fish/your girlfriend's fish will die if you skimp on the changes!
If you get a 30 gallon, as well, it will be harder to keep the water stable than it would with a 40 or a 50 gallon. The water is the most unstable part of the aquarium, and if it fluctuates, your fish will not last long. You would do better with a 40 or 50 gallon tank.
I would definitely get a good book and start reading up on setups BEFORE purchasing anything. A good book is The Conscientious Marine Aquarist, by Robert Fenner. Like DrOizo said, you should not trust your lfs. Only listen to them if you already know, from research, that the facts they give you are true.
Look at the pinned articles on the Salt Water/Marine Chitchat forum on this website, and proceed with caution. Starting a salt water tank is a big undertaking.
Good luck. 8)