Here is a GPU compairing info on each card to each other.http://www.tomshardware.com/reviews/fastest-graphics-card-radeon-geforce,3067-7.html
I would ask them to replace both cards with two 6770 not 6750 and go cross fire. Just tell them how put out you are since they don't sale Nvidias cards anymore.
Or they can give you the one 6850 for just the 8800gt. Don't let them push you around. you can always sell the 8800gt and put money down for another 6850
Or ask for your money back on bothe cards. If not then ask for the money back on the one and go on line and find one that matches the other card.
Or ask them to give you one HD 6950 for both cards. You have them in a corner, you should be able to get what you want. They have to honor the warranty so push them harder into doing you right. Anyway I wish you good luck