I don't think P5K Premium's prominent feature is meant to be DDR2-1066Mhz support as P5K Deluxe already does 266FSB 1:2 for that

From what i've been told (not much), P5K Premium is just a better oc'ing version of P5K Deluxe i.e. what Asus Commando is to Asus P5B Deluxe.

Hopefully, I should have P5K Premium soon to compare with P5K Deluxe