Agreeing with n7, but even if you set FSB and RAM to "unlinked" while picking a certain set value for RAM frequency, the board will realize a frequency according to integer arithmetic for discrete CPU : RAM ratio values.
When you dial in these settings under an "unlinked" regime, I've always found it useful to do the arithmetic that matches FSB with DDR as precisely as possible and for a chosen ratio. Here's an example.
EXAMPLE:
You have a CPU spec'd for 1333 FSB -- CPU_FSB of 333 and multiplier 10 -- and you want to OC it so that it runs DDR2-1000 RAM at its rated spec. But you can't get this CPU to 2,000 FSB, so you choose a ratio of 4:5.
Then 5 * X = 500 [calculated as half the DDR speed]
and X = 100
You would then attempt to OC the processor at its stock multiplier so that:
4 * 4 * 100 = 1,600 FSB
That is, the quad data rate factor is 4, you need an additional factor of 4 for the 4:5 ratio, and you would run up the front side bus from 1333 to 1600 so you could achieve the RAM spec of DDR2-1000 at a 4:5 ratio.
If for instance the CPU multiplier is 10, you are starting from a CPU_FSB (a fourth of the [QDR] FSB) of 333 and 3.33 Ghz, to over-clock to 10 * 400 or 4.0 Ghz.
In other words, with your own FSB OC choice, you might as well have just dialed in 981 in the motherboard setting. You can OC the CPU and FSB further, but I would choose the OC target based on the RAM DDR frequency you hope to attain. Which means . . . . "do the integer arithmetic first."
If you can't get a stable setting by running up the processor quite as far as the target, then you can either pick a different ratio or settle for less than the RAM spec. If you settle for less, you may be able to tweak the timings tighter and break even.
There are some ratios other than 1:1 (considered the best) which provide better bandwidth than others. 4:5 is among those so favored.