1720mb Turbocache would be possible with around 2-3GB system Ram installed, and you'd likely get 2GB+ Turbocache with 4GB Ram installed.
Doesnt really make any difference to gaming performance though, as another poster has mentioned, 512MB is more than enough.
More memory doesnt mean more 3D Processing power... Efficient Memory does.
An efficient 256MB card can outperform a non-efficient 512/1024MB by quite a margin.... but video card manufacturers like to try and market the fact that MORE IS BETTER to the more unwitting of us in the PC sector.
A person new to PC gaming looking for a new graphics card to play Crysis would think the following:
8800GT 512MB @ £150
8500GT 1024MB @ £50?
Double the memory for 1/3 the price? Sounds great I'll take it
Whereas a seasoned PC user who understood specs would see the 1024MB as a cheap piss-take of the lesser informed public, and probably buy a card priced according to its true performance.
OK totally off-topic but good advice to anyone new to PC's and specs