Get a GTX275, they are significantly cheaper and perform almost as well. What resolution do you normally play at? What power supply do you have? Those are what determine what you should upgrade too and how far you can safely upgrade.
Almost any decent card will be a huge upgrade over an x1900.
Now isn't the right time to be buying a card in your price range. The new HD58xx cards should be out very soon and prices will likely be changing for high end cards as a result.
That said right now an HD4890 is what I would recommend.