I think a card is probably enough since you're so new. I agree that anything more will probably not be taken in the spirit you intended it, when it comes to your co-workers.
Now, next year, see if you can up the ante & get everyone to chip in for a gift or gift card for her birthday.
I do think it's kind of weird that they don't do anything for birthdays & I think it's kind of sad. In our department, we decorate the birthday person's cube, give a group card, some do small gifts, there is always a cake, and the person usually gets taken out to lunch. I thought that was standard for most offices, but I guess I'm lucky.