Hmm, actually, wonder if I've made a mistake here....
Page 16 of the manual - http://download1.gigabyte.eu/Files/Manual/motherboard_manual_ga-ex58-ud5%28p%29_e.pdf
I'm looking at the '3-channel' '4-module' row:
DDR3_1, 3_3, 3_5 = 2GB (8,8,8,24) Sticks
DDR3_2 = 4GB (9,9,9,24) Stick
Will...