if you take a look at the "Qualified Vendors List" in your P5K motherboard manual, you will see that you can only use 2 sticks of 1066 RAM at one time, either in the black or yellow slots.
At 800Mhz you can use 4 sticks of most memory brands.
If you want a 4Gb 1066 machine, you need 2 sticks of Kingston KHX8500D2K2/2GN . There may be a more up to date list on the asus website.
The timings for my Corsair Ram are 5-5-5-18... watch out because older versions of Asus BIOS default to 5-5-5-15.
When I upgraded to the latest version of BIOS using ASUSUpdate windows tool it auto-detected the timings properly.