Technically, what happens is that your RAM can send data faster then the CPU can deal with it (or vica-versa, although that doesn't happen often...), so it has to wait for the CPU to send a signal. This does take some time, but it is usually minimal.
This is how you get 1:1:
Take your FSB (800) and divide by 4: (800/4 = 200)
Take your RAM speed (667) and divide by 2 (667/2 = ~334)
To run at 1:1, you would need to downclock your RAM to 200MHz, or OC your FSB to (334*4 = 1336) MHz.
As you can see, even though 200MHz DDR RAM would run at 1:1, the loss in speed would cause you to run slower as a result.
There are cituations where a 1:1 RAM ratio is faster (667 DDR2 with a 1333 FSB can be slightly faster than using 800MHz DDR2), but it usually isn't a huge jump.
In short, if you can get 1:1, great. Otherwise, its not that big a deal.