I had loads of problems with ubuntu and my ati x800 card. Tried all versions and no matter what I did I couldnt get X to start correctly.
It turns out that the Version of X released with feisty has issues with ATI.

Installed using text mode and got a running system up but only when I tried to log in from recovery mode as root.

I have now swapped my card for an nvidia card so fingers crossed I`ll get it sorted