Buy PC800 RDRAM. No need to buy ECC RDRAM, unless this is going into a mission critical server.
PC600 RDRAM's performance is very poor.....barely on par with PC100 SDRAM. PC800 RDRAM's performance is much better, closer to PC133 SDRAM performance, although the performance deficit for both types of RDRAM is more than made up by the dual channel i840 chipset.