[Graphics Cards] Upgrade Question

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.

1920X1200 and I think I got a 800 watt power supply

The nVidia 275 or HD 4890 is more than needed to drive 1920.

As you can see in the table, the upgrade is *huge*:

http://www.tomshardware.com/reviews/best-graphics-card,...

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.

They say pci express 2 cards normally work on mobo's which are pci express 1.0... would this still be a good option to get or would the pci express 1 bus limit the card too much?

should be fine, though wait for the HD5XXX series see what prices can be found

Wait for the Radeon HD5xxx series card release and you will be benefitted, I think... because it will lead price drops on the current cards...