That's odd, I ran an 8800gt for a while before upgrading to the HD 5770 I have now and I never had the issue of not been able to display in that resolution. I guess 1 of 2 things could be happening: 1 is that there is a conflict between ATIs drivers and the nVidia drivers; 2 could be that wow needs a repair.exe run to clear out any settings relating to running under the different cards.