MSN Messenger- picture messages will use more data than text
Checking emails - downloading attached files and pictures will use more data than just reading the email
Randomly using apps - data usage will vary by the particular app. Some apps are data intensive. Others require no data at all. Some apps are little more than mobile friendly web pages; these can use data just like you would if you were using the Safari app.
The 2GB data plan is 10 times (rounded) as much data as the 200 MB plan. I'm not sure if this is what you were asking, though.
Really depends on your usage. If you still aren't sure how much data you will use after looking at AT&T's data tool, I would recommend that you start with the $25/2GB data plan. You can always lower your plan at any time if you find that you are only using 200MB per month or less.