make sure that you have the extra 4 power connector thing hooked up.... cause if its a newer mobo you need it.... i made that mistake earlier with this computer and i thought it was dead but it turns out i needed to connect the power supply=P oops!=P
Did you wear an ESD strap or take the proper measures against static????? Also take the 6800 out and put your old AGP in to see if that works. If it works then you can eliminate the theory that your mobo is fried.
Im not sure for the ultra but the 6800 GT requires power connection without anything else plugged into the same connection. I ran into a similar problem and it turned out that the agp slot had somehow gotten fried