The new 2GB modules should have the same or lower operating voltage and at least the same or better memory clock speed and timings.
They should be installed on the GA-Z77X-UD3H in the following fashion:
DDR3_1 - G-Skill 4GB (1333)
DDR3_2 - G-Skill 4GB (1333)
DDR3_3 - New 2GB (1333 or faster)
DDR3_4 - New 2GB (1333 or faster)