Rotate with Radeon 7000

[Emmanuel Seyman]

I'm experimenting with a SyncMaster 171s monitor and a Radeon 7000
video card. I'ld like to try the screen's portrait mode but info
on how to do this is sparse (or maybe my brain isn't capable of parsing
it at 4:30am).

Do I need to use the Rotate option in /etc/X11/XF86Config?
Or the RanR option?
What's this xrandr program?
Does it do anything but output error messages?

Emmanuel