I would get the HZ's. I guarantee you will not see any advantage with 6GB of RAM. I'm not an expert on how all that works with the memory and dual channel but I would say it is a true waste to get 6GB of RAM. Alot of people think 4GB is a waste but I dont like to discourage it really because I was considering it for a while until I got to college and don't have the money for it anymore.
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EVGA 780i SLI
QX6700 @ 3.3GHz cooled by custom WC loop
EVGA 8800GTX @ 620/1000
G.SKILL HZ's @ 1100
2x WD Raptor X
WD Black 1TB
Extremegamer Fatality Pro
Thermaltake Toughpower 700w
Thermaltake Armor