First I did an upgrade on my x64 installation of Feisty. It worked mostly, but I lost graphics. This helpful window came up and asked me for monitor and graphics card specifications, so I entered them but only got 800x600 resolution.
My system is:
AMD Athlon x64 AM2
nVidia GeForce 7600GT 256 Mb
Compaq S720 monitor (which btw isn't listed, but S710 is so I chose that one)
I actually didn't even try to resolve this. I just downloaded the CD and did a clean install (I have /home on a separate partition so it's the easiest way anyways). The clean installation worked great immediately, but since I could only choose one I answered on the upgrade issue in the poll.