I believe there's some sort of 20A limit for each 12v rail (to prevent them exploding, in the more dramatic cases), so for cards like a GTX260 (which requires about 22.5A I think) which needs two PCI-E connectors, each PCI-E connector will be on a different rail. And then things like CPU and hard drives can take up the rest.
So, I don't think they can "share" amps, but you see what I mean.
I'm not entirely sure that's the case, I may be wrong, I ain't an electrical engineer
Edit: I shouldn't have watched an episode of Family Guy while writing this, hefe beat me with much more accurate sources