Quick question... DO we know how much of that 8 gb RAM that will actually go into the games? Wii U have 2gb RAM, but only 1gb is used for the games... Considering all the features Sony has implemented into the PS4 (playing games while downloading, sharing videos etc) I think it's safe to say that games won't use the whole RAM...
1GB, 2 at most I think - PS3 OS takes only 50MB.
EDIT: Oh, sorry, I answered backwards - so that would be 6-7GB