I think it's a question of what you're prepared to put up with and how big you're going to be viewing the output. That said, in general I try to stay under 1600, but 3200 is definitely usable with a little bit of cleaning up. By 6400 things start to get a bit ugly, but it's OK for small images. This will obviously vary a little depending on subject matter and PP.
I don''t know if everyone would agree with this but I think that one of the nice things about the 60d is that the noise is less ugly than on some of the older cameras - to me it seems a bit more random, with less pattern noise.