Well, I've seen many guides to put retail snow leopard working on P5K but none worked for me, so I got another way.

Here it is:

1. Download SnowOSX v1.5 (google is your friend, SnowOSX is a distro like iDeneb and iATKOS)

2. While downloading SnowOSX, lets prepare the additional files we need. Download this archive and extract it to a folder inside a pen drive or external hard disk.

3. When SnowOSX download is done, burn it to a DVD (single layer is fine).

4. Boot from the DVD.

5. Open disk utility and format your desired hard drive. Also, you can't have other systems installed on that hard drive, only Snow Leopard because if you have, it will not work. Choose GUID when partitioning. You can use MBR too, but I didn't tested if this works with it so GUID is preferrable.

6. Close Disk Utility and open Terminal. Write:
cd /
sh MakeDSDT SnowLeopardPartitionName

When it asks to press any key, press enter (I don't know if it was a problem of my pc but only enter skipped that)

Then, when it asks for Darwin, Windows, etc. Write 0 and press enter. (it chooses Darwin).

7. Now you are done to start the install. Install Snow Leopard on the desired partition and leave all the default selections (don't click Customize, just Install).

8. It will take some time, after its done reboot.

9. Now, let Snow Leopard start and.... Kernel Panic!!! Its easy to fix, don't worry.

10. Insert your pendrive/external hard disk, boot from SnowOSX DVD again and open terminal. Write:
cd /Volumes/PenDriveName/FolderWhereYouPutTheKexts
cp -R * /Volumes/SnowPartition/System/Library/Extensions/
diskutil repairPermissions /Volumes/SnowPartition/System/Library/Extensions

11. Reboot. When you come to the Snow Leopard bootloader press F8, write -v and start.

12. Enjoy Snow Leopard :P Now you only need to set EFI strings for your graphic card and its fully functional.

Note: You only need to start with -v the first time.