Rachel,
Try to see where you are having difficulty in getting what you want. For most purposes, frame wide and also closely. Take two pictures from the same position. That will allow you to see the context of what is important when you are in front of the screen. Are you wide enough? Can you get what you want to fill the screen. That's a balance that's very difficult and we must always struggle with.
When you realize where your lenses are holding you back, only then should you buy more.
An example, if you want to take pictures of a birds through the wire of the enclosure, a longer lens with a wider aperture would allow the birds to be in focus but the wire would not! So if that is what you want to do, a 70-200 f 4.0 L IS used wide open would be perfect and the IS would take care of the increased movement sensitivity of working with longer lenses.
However, without getting limited by your lenses, I'd not buy anything else!
So, are you restricted for what you want to do?
Happy 2008 in photography and spousal photography!
Asher