First check what kernel you have:
to determine if you have a i386 or amd64.
NOTE: In case you have an new ATI graphics card you will very probably need to update to the latest drivers first for your Xorg to work with the newest kernels!!!
you have to install the 3 .deb files from http://kernel.ubuntu.com/~kernel-ppa/mainline exactly in the following order:
1) Install linux-headers-XXXXX_all.deb
sudo dpkg -i linux-headers-XXXXX_all.deb
2) Install linux-headers-XXXXX_amd64.deb
sudo dpkg -i linux-headers-XXXXX_amd64.deb
3) Install linux-image-XXXXX_amd64.deb
sudo dpkg -i linux-image-XXXXX_amd64.deb
After that just reboot.