yes it will cut into x8. But you'll not notice the difference between x8 and x16 bandwidth even with a 5870!! so don't worry.
I have another question ... the motherboard (ASUS P7P55D-E) only really have USB3 and SATA3 if I change the pcie to x8? or USB3 and SATA3 are allways there and "yo level up" only increases a little their performances?
Why do you say I will not feel the difference between x8 and x16?