Rocks, your explanation is great and I would like to add some info. From my knowledge it's the same calculation for Dual or Quad core CPU...

The following formulas maybe a more simple idea for the same thing:

(1) FSB x Multiplier = actual core speed, e.g. 333Mhz x 9 = 3000Mhz or 3.0GHz

(2) DRAM Frequency x 2 = actual ram speed as printed on the packing of the ram, e.g. 333.33Mhz x 2 = 667Mhz , 400Mhz x 2 = 800Mhz

(3) In CPU-Z - Memory page, you can find FSB : DRAM ratio, e.g 1:1, 2:3, 4:5

Case study:
If you have a E8400 running at 3.0G (3000Mhz) with multiplier = 9 and a pair of 1G 800Mhz ram, default "Link and Sync" will give 1:1 ratio to you, which means
FSB x 9 = 3000Mhz
FSB = 3000Mhz / 9
= 333.333Mhz <- from formula (1)

FSB : DRAM = 1:1 = 333.333 : DRAM <- from formula (3)
Thus DRAM freq is also 333.333Mhz

So actual memory speed is 333.333 x 2 = 667Mhz <- from formula (2)

Notice that you are not running at the speed printed on the box (800Mhz), right? Pretty upset uhhhhhhhh...... let's overclock the memory!

From formula (3) you need DRAM freq x 2 = actual memory speed
DRAM freq = 800Mhz /2 = 400Mhz

FSB : DRAM = 333.333 : 400 Mhz = 5:6

In Bios - FSB & Memory Config,
You may either choose 5:6 in "Linked" mode and not sync, but 5:6
(actually I forgot whether there is option 5:6 for you to choose)

OR (I prefer the second method)

Choose "Unlink", key in 1333.333 (333.333 x 4) for the upper box.
800 (400 x 2) for the second box and hit [Enter] twice. The gray figure will show the accepted number for memory speed. In this case, 800 will be accepted since it fits 5:6 ratio.

The so-called second method above is quite convenient in S2F bios since you can enter FSB x 4 & memory speed directly in Unlink mode and let the bios decide which ratio to run. But the overall theory should be understood so that in the future you can ride on any board~~~

If you change the Multiplier manually to 7.5 and FSB = 400Mhz, you can run Linked and Sync (1:1) at memory speed 800Mhz~ Notice the change of multiplier which affect your calculation. You should know why now~!

Hope this also help~~~~~~