Javiroces --
I have the same iMac 8,1 and video card as you do. I'm running Snow Leopard 10.6.8
Are you running 10.6.8? If so, TK111 is right and you must have OS X boot in 64-bit mode to prevent kernel panics. (At least, this is what I've read on these pages.) Snow Leopard defaults to booting in 32-bit mode. (I believe Lion and Mountain Lion default to booting in 64-bit mode.)
If you do have Snow Leopard, you can boot into 64-bit mode by holding down keys 6 and 4 during startup, but I think it is better and easier to change the default boot mode to 64-bit. Follow Tardigrada's instructions in the post just above yours, or read the excellent links he provided for more information.
Now that you have a working system, I would use the
sudo systemsetup -setkernelbootarchitecture x86_64
method, then install the kext fixes again. That should eliminate the kernel panic (assuming you're running 10.6.8).
I've been running in 64-bit mode for over a year, with the kext fix, and haven't had any screen freezes in that time. There no problems introduced by running 64-bit mode either -- everything works as before.
Good luck and hope it works as well for you.