You were thinking of getting another X800XL and doing exactly what with them? The Ultra-D is NOT an SLI board first of all (the SLI-D is), second of all you can't SLI ATI cards (you need to do that on an ATI Crossfire board) and third of all you need to get a Crossfire master card with ATI.
So if you were thinking of running two regular X800XL's together, then there's no way that is possible, not even on an ATI board. Ditch the X800XL (sell it on FS/FT) and get an X1800 or X1900 card.