I wouldn't expect a substantial increase by any means. That way if you do then it'll be cool, opposed to expecting a lot of FPS gain and seeing nothing.
I'd say no more than 5Fps, 512mb is enough to handle the games as it is. Going from 256mb to 512mb has a drastic improvement but 512x2 isn't really so.
The games do load faster, but like I said I wouldn't expect a huge increase although some increase in smoothness will be there