Not much. When you sign in PSN updates the message box display, What's New content, Playstation Store XMB display and your friends status; so there is probably at least a few hundred thousand kilobytes being downloaded shortly after sign-in. The exact amount varies based on a number of factors including how many friends you have in your friend list, and how long it has been since you last signed in. Text chat doesn't use a significant amount of bandwidth on the PS3, any more than it does on any other platform. Playing games online doesn't use a lot of bandwidth either, but it requires a low-latency connection to work well. The main use of bandwidth is downloading content from the Playstation Store, and video chat.