Generally, the best way to choose a piece of equipment is to think about what you've shot and what you haven't been able to shoot because of an equipment issue. If there's a piece of gear that will make your life easier and/or images better, then it's a good thing to consider. Of course that's easier to say when you have been shooting for awhile and have a few lenses, etc...

Do you have an external flash? An SB800 would be pushing your budget but may be a good addition, and there's always the smaller (less expensive) SB600. I can't think of a lens that would really be great in that price range other than the ~$100 50mm f1.8. Maybe this lens and a good polarizing filter like Nikon's or the Hoya HMC series.