Currently running 2 17" monitors.
The second sits on top of my TV. and is used for MSN/XFire/Winamp/uTorrent/etc. The lower resolution makes reading the conversations etc. easier at the farther distance.
http://img362.imageshack.us/img362/6181/desktop1dh4.jpg
I borrowed the Vista disks from a friend, so I'm just using the 30 day trial of Ultimate.
I'm considering buying it.