There is always one part of the system that speeds ahead of the rest, and more than one part that slows down the others. This is evident in the front side bus speed being much faster than the memory bus speed, and by the fact that PCI technology is still behind the FSB in performance by a huge amount.
That said, you can buy Rambus-compatible motherboards that allow 800Mhz memory modules, and AGP 8x boards and graphics cards that run at 533Mhz, so these two combined offer a vast amount of power to high-end users such as gamers and graphic designers/3d modellers. Rambus memory is still very expensive though, and not widely used.
If you want the best performance out of a 533Mhz bus without spending on Rambus, get PC3200 memory well over 512Mb, and an AGP 8x graphics card. You will find that the system runs smoothly enough! During gaming, for instance, the memory bus is supplying data to both the processor and graphics processor, and not engaging either 100% of the time. The processors are often still trying to process the data while more is being supplied during high-end gaming.