New Computer

Im upgrading to Dell's XPS 210 for the holiday season, and will be purchaasing the most expensive unit. Im wondering whether its video card ( the 256MB ATI Radeon X1300 Pro) is powerful and new enough to run FSX on high settings. Advice appreaciated! thanks

Yes and no.
It'll do a good job for you...now.
The DirectX 10 cards will be out when Vista comes out.
Between the two of them the sim should be beautiful.

Im not planning to repoace the 256MB ATI Radeon X1300 Pro that the XPS unit comes with. Will it need to be replaced once Vista comes out?

DirectX 10 comes with Vista and a card that capable of using it will be worth while.
You can keep what you have but it won't be able to use DX 10 and still will use DX 9.0c. that you have now.