Video memory in general really never gives much of a performance boost unless you were running out of VRAM in the first place.
You see a lot of people buying the 1 gig of VRAM versions of GPU's over the 512 meg versions thinking that it will make all their games faster, when really most games don't use more than 200-300 megs of VRAM at once, at most.
It's kind of like how if you have 8 gigs of RAM, and you buy 16 gigs, unless you're really filling up RAM, you're not going to see much difference...