I agree with everyone who has said to wait.
If she is someone who can't wait, the 3GS is a significant upgrade over the 3G, especially if you're a power user. However, most of the improvements are "under-the-hood", so you can't see them, such as new native apps and new features.
To me, the difference is huge, and I did have the 3G model. It has a much faster processor, connects to the Internet faster, downloads apps a lot faster (AT&T service permitting), but the 3G model is no slouch either. How would you gauge your girlfriend's "need for speed"? Would you say that she is a "casual user"?
I do understand the whole "gotta have it now". Having said that, I got mine in mid-December, slightly more than three months ago, and if I were contemplating buying an iPhone today, I would wait.