The 7800gt overall performs better than the 1800xl and it costs more than a GTX! I know you can overclock the xl but you can overclock the gt as well. Did I mention its costs more than the GTX? When the price comes down to a competitive level and you all ready have a gtx there is absolutely no reason to get the xt. If your just buying the card it will just depend what games you want to play. it will be a toss up. If your wanting to get the next step under them and if you already have a gt, there is no reason to get the gt. If your just buying a card you should get the gt.
But if it were me by the time the prices come down to a competitive level something better would prolly be coming out right around the corner. I would prolly wait or that for better DX10