Limit widget to certain sidebar?

I have multiple “sidebars”, but not all of them are the same size. Not all widgets fit in all sidebars (for example, I have a “footer sidebar” where the client can place custom widgets, but they are wide, and don’t fit at all in the “real” sidebar). I want to give an indication when a widget is placed in a sidebar where it would not belong (change the header text color, for example). What would be the best way to do this?

How do I transition multiple installations to a single Multisite installation?

I have a client who runs 8 different blogs, currently on 8 separate installations of WordPress. I’m trying to encourage them to move to a single MS installation instead to make it easier (mostly on me) to maintain the different sites, themes, and mix of plug-ins. Unfortunately, I shied away from MU for the longest time, and now I have no idea where to start with MS.