Does the FPS increase when you lower the graphics settings?
Your Crysis performance look something like this chart?
And you like the FPS result on the last line of the chart better?
Your GIGABYTE GA-880GA-UD3H 880G motherboards 2nd PCI-e x16 slot runs @ x4.
Not ideal for a CrossfireX setup but 2x 6870 will work.
What do you have for a power supply?