GA-Z68XP-UD3P (rev. 1.3) has the pcie 3.0 support, the GA-Z68XP-UD3P (rev. 1.0) doesn't have.
I don't know what revision your mobo is, but anyway it won't make a difference in terms of performance pcie 3.0 vs pcie 2.0 : http://www.hardocp.com/images/articles/1341219566Ew8yr7oTVd_2_3.gif
The important thing, is to update the mobo with the latest bios with a sandybridge cpu.