Prior to installing fglrx, you were using the open source drivers which support KMS. KMS will provide you with console resolution that's the same as your monitors preferred mode. After installing fglrx, you are using the vesa framebuffer driver, which does not support non-vesa modes.
You could try using uvesafb, but I have no experience with that. There is nothing in xorg.conf that you could try as xorg.conf only impacts X, not the console.