Boot into 9.04 but nothing happens

[claymater]

I installed Ubuntu 9.04 over the network upgrade, and when it was installing it gave me a message that said something like "Your graphics card driver is not supported in this installation of ubuntu, continue?" So I pushed "yes" (I have a ATI Radeon x1300)
But now when I boot into ubuntu it goes fine till the log-in Screen, then goes blank.

Any suggestions on how to fix?