If your system uses DDR333, getting DDR400 won't do any good, UNLESS you get RAM that can handle low latencies. And then you'll be playing with a lot of settings in bios to try and customize everything to get every ounce of power you can out of it. That's stuff that's normally reserved for the overclockers/enthusiasts. It can get ugly since there's so many brands and they all perform differently. DDR vs RDRAM is a no-contest. RDRAM has almost zero support (in the mainstream) since Intel changed everything to DDR. I'd rather get a new motherboard, cpu, and ram than buy more RDRAM. The 800 mhz fsb P4s are nice, very nice. This coming from an Athlon owner.