How to match RAM speed with Intel CPU FSB

There are currently 3 types of RAM still in use.

DDR: Double Data Rate:

• The memory chips and the memory bus run at one speed.

DDR2: Double Data Rate 2:

• The memory chips run at one speed, the memory bus runs at twice that speed, and the bus can transfer two pieces of data at once.

DDR3: Double Data Rate 3:

• The difference is that now the bus can transfer 4 pieces of data at once.

The rated "speed" of RAM is how many million transfers it can perform per second.
DDR2-800 can do 800 million transfers a second at peak bandwidth usage.

Meanwhile, Intel likes to confuse you, so the FSB of a CPU is actually 4x what the actually speed is.

Let's take an E8400 Wolfdale dual-core, shall we? It has:
a multiplier of 9x
a FSB of 1333mhz
a speed of 3ghz.

Now, 1333x9 is 12ghz, which is definitely not the speed of your CPU (sorry)
To get the actually speed of the chip, divide the FSB by four.
1333mhz/4 = 333mhz.

Now, let's try again:
9x333=3.0ghz (2997mhz if you're picky)

So you need to use DDR2-667 RAM with this CPU at least. (667/2 = 333)