I'm no expert on keyboards, but I do know that if your girlfriend already plays piano she would probably appreciate a keyboard with weighted keys. These are heavier and more expensive than keyboards with unweighted keys, but more closely resemble the kind of experience you're used to if you play piano. It all depends on what she wants to do with it. If she's just going to play at home, then portability isn't an issue and weighted keys will be fine. If she's going to take it places, you might want something lighter.