If you look at the cpu/overclocking forum, lots of poeple said Samsung PC800 RDRam is a good way to get to 1066Mhz, they seem to max at around the low 1100s.
Since you're looking for 512MB sticks, you may have a tougher time doing so. However, people here have recently posted that they have gotten high density 256MB sticks to 1066Mhz (which I am assuming they use in 512MB sticks).
Googlegear.com sells Samsung 512 sticks of rambus for $200+, 256 sticks for around $80. I recently ordered a pair of 16 device (low density) 256 sticks myself.
But then you must know that nothing is a garuntee.