looked at the game. looks like skyrim meets the 24th century. the 8gigs, I think your 6 should be fine but the more you have the more the system will use.
what motherboard do you have? some of those boards had 4slots so adding another stick of the same stuff ( if applicable ) shouldn't be a problem. would be less expensive.
the difference in 1333 vs 1600 might mean 5 or 9 fps loss..... maybe......... if you went with the 12gb. your call. ................... find the game interesting. a whole "universe" of usable stuff and huge futuristic environment. must have been a monumental task for the level editor crew.