Onions are also bad.

When I was little, I would feed my Lab grapes sometimes because I didn't know any better. I only gave her a couple at a time and nothing ever happened. I would imagine different dogs would react differently to it though. Sort of like with chocolate - it takes different amounts to become toxic to different dogs (depending largely on size, but also other things).

To be on the safe side, I wouldn't feed any grapes at all.