IntroductionAMD are moving on to a 166FSB for their XP processors in the very near future (XP2700 and XP2800+ already are, but are not widely available). Intel have recently ratified their DDR motherboards for official 166MHz memory (DDR333 or PC2700) use. With these facts in mind, you'd be forgiven for thinking that PC2700 memory was the best you can currently buy.
Led mainly by the SiS648 chipset that offers unofficial DDR400 support, memory manufacturers have been trying to outdo each other with faster and faster DDR modules. The summer of '02 saw reasonable widespread availability of PC3200 from, amongst others, Corsair, Mushkin, Kingmax, Winbond, and Geil. This memory, operating at 200MHz with varying timings depending upon how brave the manufacturer felt, gave enthusiasts a real chance to push their systems to the limit, safe in the knowledge that system memory wouldn't be an impeding factor until at least 200MHz.
The core architecture of the Athlon XP processor dictates that it works best when run with memory running synchronously to the CPU's FSB. It's double-pumped FSB tallies perfectly with DDR running at the same speed. So, PC3200 memory would allow you to run your processor, once unlocked, up to 200MHz with varying timings. Running memory at speeds greatly in excess of the system's FSB would prove to be a largely futile exercise.
The P4, on the other hand, with its massive quad-pumped FSB, simply wants for more bandwidth than the very best modules can supply. After all, at 133FSB it can use up to 4.26GB/s of bandwidth. That would need some hypothetical PC4266 memory for full bandwidth saturation, assuming excellent efficiency.
Corsair launched some excellent PC3200 memory recently (XMS3200 C2) that many an enthusiast, including yours truly, ran at 200MHz+ with strict timings for both platforms. We can see a need for faster DDR memory for the P4 with its seemingly insatiable bandwidth requirement, but faster RAM for the AMD platform is less important due to chipset limitations imposing themselves at 200FSB+. Still, faster is undoubtedly better.