Unfortunately there seems to be regression in the Ubuntu support for Radeon Mobility 7500 in 10.04. I have this graphics card in a Thinkpad T42, and it's causing me much frustration attempting to have it perform as power efficiently with decent graphic speeds, as delivered in Ubuntu 8.04. It's especially power hungry in suspend sleep.
KMS might be the source of your display problems. You could test and see without rebooting, by running this from a virtual console:
If this helps, then you can commit the changes by using your favourite text editor (I'm using vim as an example) and making this file:
$ sudo stop gdm
$ sudo modprobe -r radeon drm
$ sudo modprobe radeon modeset=0
$ sudo start gdm
Within this file add these lines:
$ sudo vim /etc/modprobe.d/radeon-kms.conf
Then to commit these changes, rebuild initrd.img~ by using:
# Prevent KMS from loading
options radeon modeset=0
sudo update-initramfs -u
Some more KMS details are available here:
Hope that helps for starters.