Any loss of 'dual channel' performance from running 3x2Gb of RAMs can be easily overcome by OC'ing the NB/IMC (generally simple process).
IIRC there are quite a number of 3x2Gb kits of various speeds approved for that motherboard on the Gigabyte QVL list. I would highly suggest you consider them.
As always, if you add an additional stick verify same capacity, brand, speed, and chips as your originals to avoid any problems.
2 x HD5770s will be a stout gaming rig