I finally found the solution for running the nvidia grafic driver with multiple kernels.

Here are my steps:

1. boot with the new kernel.
2. go to the following directory : /lib/modules/OLDKERNELVERSION/kernel/drivers/video
replace OLDKERNELVERSION with your own specific settings
3. make a copy of the old driver: cp nvidia.(k)o nvidia.(k)o.old
4. run the nvidia installer
this will create the new driver in /lib/modules/CURRENTKERNELVERSION/kernel/drivers/video
5. in the same directory as mentioned under 2 do a rename to the original name
mv nvidia.(k)o.old nvidia.(k)o

Now you should have a working driver with both kernel versions.

Great. will try that out the next time I reboot.

I've heard about a -K option to add in the command line. Did someone used it?

So, you are essentially hiding the old module from the Nvidia Installer so it can't be deleted.
I guess you have to do this each time you run the installer for a new kernel. Couldn't you just remove write privelages?

After a recompile I just

sh NVIDIA-* -K
(BTW that's a capital K)

and all my kernels will have nvidia. I have 2.6.8.1, 2.6.7 and 2.4.26 still ready to run under GRUB.