Not much of one, no. 4GB->6GB would allow you to, say, leave a few apps (Chrome, etc) open when opening a rather demanding PC game without any performance loss, and the 460 to 560 is a mild bump in speed at nearly all resolutions and detail settings, but to spend full retail price on this stuff? Naw. I suggest you hold out for the next cycle of nVidia cards to come along. 4GB and a 460 will still take you pretty far if you're running one 24" monitor or less and aren't trying to multitask half the apps in your Start Menu.
'Course, if you are, might as well go to 8GB instead of 6GB...