If you've got a Nvidia graphics card you'll need to install Nvidia 3D drivers with 2.6 kernel support after booting into 2.6 so download them and configure your /etc/X11/XF86Config-4 file if necessary prior to rebooting.
Get the latest official Nvidia driver with kernel 2.6 support from here:
http://www.nvidia.com/object/linux.html
Installing the nVidia 3D driver:
1) You'll most likely need to have the nvidia installer compile the driver so make sure you have the 2.6 kernel source installed in /usr/src/linux (if you're following this document then you already do). Also, remember that the linux in /usr/src/linux should be just a link to the kernel-source directory for your running kernel.
2) You'll need to logout of X before installing the driver, do this by opening a terminal and typing init 3, or init 1 (mine was init 1 but most ditro's use init 3 for logging in without X). Next, login as root, cd to the directory where you downloaded the nvidia intaller to, type ls and hit enter, type sh NV, hit the [tab] key then enter.
Configure XF86Config-4:
If this is your first time insalling the official nvidia driver then you'll need to follow the steps below. It's much easier to do these steps while logged into KDE/Gnome/etc... before you log out of X to run the installation routine, otherwise you'll need to know how to use the non graphical Vim text editor.
1) Open a console and make a backup of your XF86Config-4 file with:
cd /etc/X11
cp XF86Config-4 XF86Config-4-orig
2) Open your /etc/X11/XF86Config-4 file with a text editor. Under section "Module" make sure you have the following line:
Load "glx"
If you have these lines, remove them:
Load "dri"
Load "GLcore"
Now scroll down to the "Graphics device section" and change:
Driver "nv"
To:
Driver "nvidia"
3) Make the above changes, save, exit, reboot to your new kernel and cross your fingers.
Install Error's:
You must have the appropriate kernel source that matches your running kernel and a .config file in /usr/src/linux or nvidia won't compile.
If you receive any errors while running the nvidia installer then check this file:
/var/log/nvidia-installer.log