3GB+ seems like an exuberant amount of RAM for the OS features they have announced. It seems they're intent to see what OS features stick on Microsoft's system, copy the successful ones and perhaps reduce the OS footprint down the line.
Microsoft said their OS doesn't need 3GB at the moment, but they reserved that much space to be able to expand OS featureset in the future (unlike 360, where the 32MB left them with no room to move). Which means both consoles could do some pretty cool shit in the background years from now and if they can't come up with anything cool they can make more space for games.
Either way, 5GB on both system is like 12 times more than the space available to PS3 developers. Plenty enough to fit game logic and assets far, far beyond last gen.