This is going to be a very very crude explanation but: Intel rates their front side bus or FSB by quad pumped values.
On PC's, it goes like this:
A 2.4GHz E4500 core 2 duo allendale has an 800 MHz FSB quad pumped. It's 2.4GHz comes from the 12X multiplier so 12 x 200 mhz = 2.4GHz.
RAM can be set to linked (which is a 1:1 ratio) to FSB or unliked (and arbitrary multipliers). Ram, such as the DDR2 ram here, is the base FSB * 2. So if the processor is running at 200FSB (or 800FSB quad pumped, as advertised here), and RAM is linked in a direct 1:1 ratio (or 2.0 multiplier in other words), the RAM is running at 2 * 200 mhz = 400Mhz, or DDR2 400.
If multiplier is 3.33, then 3.33 * 200 = 667Mhz or DDR2 667.
For example, my brother's PC has a Core 2 Duo E6550 2.33GHz running at 1333FSB (or 4 x 333). Thus 2.33GHz / 333 MHz = 7 x multiplier. When overclocked to 450 FSB, the computer is running at 7 x 450 = 3.15 GHz.
The RAM came stock at DDR2 800 but when set to linked 1:1, at the overclocked settings, the RAM is running at 2 x 450 = 900 MHz or in other words, it is overclocked over its originally rated speed.
I know this pertains more to PC's and their BIOSes than Macs but that's the gist of it. In a nutshell: Lower rated RAM doesn't mean lower speeds at all unless overclocking is involved, and that's not involved here at all.
