How to match cpu to memory speed

I'm looking to build a new computer and would like to know how to match CPU speed to memory speed. To get the best performance the speeds should be matched. Mismatching just waste money and performance. Now that there is so many new things there is a lot to learn. I'm looking at AMD Phenom CPUs and DDR2 & DDR3 memory, but the speed of each doesn't indicate which should be used together. Anybody have any imformation on how to match these items? Thank you, Steve

The rule was for Intel LGA 775 builds.

For AMD, you just need DDR2 1066 or DDR3 1333. Anything faster than that is wasted with the current memory controller.

That's not to say you shouldn't buy 1600Mhz DDR3 now if the price is right. DDR3 will be around for a while and maybe some future build will make better use of faster RAM.

It's ALWAYS a good idea to get the lowest timings and lowest voltage you can at those speeds however.

Stop making it more complicated than it needs to be

There is no speed matching needed with AMD CPUs. Get either DDR2 1066 or DDR3 1333 (or higher if you like, but it won't make the computer faster).

Older AMD CPUs only use DDR2. The newest AMD CPUs use either DDR2 OR DDR3. For those, like the X4 955, there is no real speed difference between the two memory types. You can use either DDR2 1066 or DDR3 1333 and get the same results.

With LGA 775, the "ideal" memory speed is tied to the FSB x 2. So if the Front Side Bus (FSB) is set to 333Mhz, like it is with a Q9550 for instance, then all you need is 667Mhz RAM (333 x 2). Now, 800Mhz RAM is pretty cheap, so just go ahead and buy that and run it at 667. Won't hurt anything.
Now, if you plan on overclocking that Q9550 a lot and raising the FSB to 420Mhz, the you will need memory that can operate at at least 840Mhz, so you might want to buy 1066Mhz RAM.