At what resolution would you start taking advantage of 1gb vram?

^
Agreed your sugested res wont have an issue with most games, The new GTA is said to use plenty of Vram and could be an exception but generally its as S/S says

Mactronix

I saw adventage with GTX 285 2 gigs .... at 19x12 ..in a new games ...

A little dated, but the following shows performance games between a HD 4870 1GB vs. HD 4870 512GB

http://www.xbitlabs.com/articles/video/display/radeon-h...

Edit:

thanks. would a 512mb max out 1680x1050 full aa/af, given that the card has the ability to do it. (gtx 295 mars edition but with only 512mb ddr3 256 bit as a sample.)

It really just depends on the game. Generally having more than 512MB of Ram provides a visible benefit at resolutions above 1680x1050. With AA and AF then it's more like 1680x1050 and above.