I think it depends on your girlfriend. The best gift is going to be something tailored to her specifically. A good way to get some ideas is to look at what she buys for herself, and what her most treasured items are. It's different for everyone. I'd try to get her something she has always wanted, but maybe hasn't got around to buying.
Or you could go with jewelry. $300 is enough to find some really great items, but again, you want to make sure they suit her specifically. IF you're going to go the jewelry route, take note of what style of necklaces and earnings she seems to be fond of, and get her something that fits into that, but is of higher quality.
That's what I'd suggest. Some guys think jewelry is sort of risky, but in my experience, it makes a big impression. If she likes a particular TV series, but doesn't have the DVDs, getting a collection of them can be good. My girlfriend bought me all the seasons of Millennium, which was a favorite show of mine, and that was one of my favorite gifts. It also gave us a lot of stuff to watch together. This might not be quite as good of an idea if her favorite TV series is something you wouldn't want to have to watch, though!
Ipods are pretty popular, if she's technologically inclined and doesn't have one yet. You can also get them done up with a custom message, which would make an impression.