Video card for FCPX

I'm looking at updating my iMac to the latest model. I've done some comparisons in an Apple store and outputing 1080P movies from FCPX it's significantly quicker.

My question is as there are two video options for the new machine. One has 1Gb of RAM and the other has 2Gb. does the extra Gb of Video memory have a significant effect on the output speed, as I believe FCPX uses some of the GPU's processing power

Thanks

John C

Thanks that's helpful, Also it has saved me some money