Wow...where to begin...
PC800 is Rambus DRAM, PC133 is SDRAM...
Do you know the difference between RDRAM and SDRAM? They need different chipsets, different motherboards, different everything...RDRAM is only used in P4 and some P3 chipsets. SDRAM can be found in a chipset for anything in about the last 3+ years...except the P4.
They are competing memory types...