There is no such thing as pc800 ddr RAM, the speeds of ddr available are pc1600 (200mhz) , pc2100 (266mhz), pc2400(300mhz), pc2700(333mhz) , pc3000 (366mhz) pc3200 (400mhz) , pc3500 (433mhz??).
DDR has just about caught up to RDRAM and dual channel solutions have huge memory bandwidth.
I dont think rdram goes up to pc1600? I thought only pc800 (400mhz) , pc1066 (533mhz) and pc1200 (600mhz)
You can stick with RDRAM for a little while yet and still have a really fast system.