I have got recent Samsung RDRAM PC800 no ECC and it goes to PC1066 and over easily. Not sure whether the 92mm fan on the Zalman bracket blowing over it and the chipset helps, but it sure keeps the temperature down by 20-30°C.
Unfortunately I have not managed to get my P4 2.0a to run 100% stable with a FSB of 133MHz.
So before you upgrade your RDRAM, 2 things to check:
1. Is your CPU stable at 133MHz FSB?
2. Can your current RAM reach PC1066? (you can try holding a case fan above the RDRAM with the case open to see if it helps).
As for myself, I will upgrade only when I have a CPU which can reach a FSB of 150MHz and PC1200 is out.