Does this mean that if I buy an AMD Athlon 2600 with a FBS of 333Mhz than I need 666Mhz rated DDR SDRAM?
It's smoke and mirrors. The true clock speed in this case is 166 MHz, but since it is designed to work only with DDR RAM, they refer to a 333 MHz bus.
Not that I blame AMD--Intel is even worse . They refer to an 800MHz FSB, but this is actually 200 MHz quad-pumped--that is, two interleaved banks of DDR 400 RAM, which is much more complex for, at best, a marginal improvement.
I know this is confusing, but much of today's computer marketing is based on confusion. That's how Intel sells more processors. An old joke:
Q: What's the difference between a used car salesman and a computer salesman?
A: The used car salesman knows when he's lying.