I've just installed 10.5.5 using the ATKOS_5i on a ASUS P5K Deluxe WiFi,Intel 6600 quadcore,Nvidia 8600GTS and sata Lite-on DVD-. Everything went smooth until I tried to play a DVD movie,then I got a
error code "a valid video device could not be found for playback" Anyone ever get this code? The drive
worked with the install disk and for burning the disk. Any ideas?

Yes I got that error on my hakintosh. It happened when i remove some drivers from extensions folder. Just reinstall you Mac, and right after copy your nvidia drivers to extensions folder.

Tip: to check the campability of the driver, write down the Dev ID of your video card, a then while booting in verbose mode (-v) just chek which driver loaded....
i got some roblems with my video card... ATI x1600pro... wrong DEV ID was in driver.. so there are a lot of info in this forum.. just search for Nvidia xxxxxx (xxxxxx - is your Dev ID), or you can manually edit the kext files... search the forum

P.S. good luck