I've seen PC3700
There's also DDR 2, which is still a young technology. Its only been used on some graphics cards as far as I know. On Nvidia's cards with it, it was clocked up to 500MHz (1000MHz DDR).
Actually, PC3200 (the latest standard) is traditionally only the speed of PC800 RDRAM! This is because traditional test use PC800 on the i850, which is dual channel RDRAM (hence the pairs).
New chipsets allow you to use PC3200 in dual channel mode as well. Dual channel means a pair of modules act as one module with twice as wide a bit path. So a pair of PC3200 256MB modules, which each have a 64-bit path, can act like they are one 512MB module with a 128-bit path. RDRAM was using a pair of 16-bit RIMM modules as a single 32-bit using the same technology.
So in order to get PC3200 to outperform PC1066, you need to have a dual channel chipset. The fastest dual channel chipset is the 875P, aka Canterwood, from Intel. The second fastest is the 655 chipset from SiS, which is a little outdated now.
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>