i have the P5K-E mobo and used to run it with 2gigs of Crucial ballistics DDR2 800 cl. 4,4,4,12. and roadrunner is right about reported issues with this mobo and 1066 speed mem. and that's why i played it safe and went with DDR2 800. besides, it's plenty fast with the 800 speeds!
but i recently upgraded to vista 64bit and now use 4gigs of Corsair XMS2 DDR8 cl. 4,4,4,12 mem and they both ran good on my board.
i would just go with DDR2 800 and overclock them. the two types mentioned overclock very well. but keep in mind that the (above mentioned) Corsair are designed for 64bit op systems and they don't recomend using them with 32bit systems. but i've always known OCZ, Crucial and Corsair brands to overclock well.