GPU Ram for 4K projects, how much is enough?

I have tried to answer this question many times in the past and been unable to.

I am an Art Director with MANY years in film biz. I am NOT however a colorist.

And I don't know the software well enough to test beyond "It works"

So I discussed this with someone who knows the software better than just about anybody.

He tried a GTX285 1GB vs a GTX285 2GB.

I will let the expert's words speak :

"With the stock 1G GTX285, using a 5120x2700 Red Epic file and timeline resolution, 2 parallel blur nodes runs out of GPU RAM. With the 2G GTX285, I get up to 14 parallel blurs nodes. Adding the 15th parallel blur node runs out of GPU RAM.

A 5120x2700 resolution timeline would be near a worse case scenario I think, at least on a Mac. Of course you would not usually grade a project at this timeline resolution, but you may well want to render it out at the native file resolution. I think 14 parallel blur nodes is plenty of headroom for most normal grading. I doubt anyone would have an issue with that. So for high resolution timelines, the 2G card would definitely be desirable over the 1G card."

So somewhere between 1 GB and 2 GB seems to be the current "Magic Number".