DDR stands for "Double Data Rate", so DDR memory runs at twice the speed of the FSB. Back before DDR, if you RAM and your FSB ran at the same speed they were said to run "in sync". With DDR the RAM is running in sync if it is twice the speed of the FSB.
On a DS3 (which is my mobo as well) you can set the memory to run at 2x (i.e. in sync) or you can increase the memory multiplier in increments of 0.5 (i.e. x2.5, x3.0, etc). This is so that, e.g., if you FSB is 266, but you have RAM rated at DDR800, you can get it to run faster than 533MHz (i.e. twice the FSB of 266).
However, running the memory asyncrously actually has quite a significant performance hit associated with it. For example, RAM running at DDR667 in sync with the FSB will actually be performing at speeds comparable to DDR800 that is running asyncrously.
The default voltage for the RAM (the "VDIMM" setting) is 1.8V on the DS3. Your GEIL RAM is almost certainly specced to run at higher VDIMM than this. Double-check for your particular RAM, but often performance RAM needs 2.0V or 2.1V.
I only just started trying to overclock my Core 2 and DS3 last night, but I have got it to 350FSB (CPU @ 2.8GHz with a x8 multiplier) without having to increase any voltages - except the VDIMM, which I have set to +0.2V (i.e. to give 2.0V to my Corsair Dominator RAM, which actually is specced for 2.1V I think in any case).
With those settings, I've got the RAM running in sync at a 2x multiplier. This means it's running at effectively DDR700 at the minute, but because it's in sync it will perform just as well (or better) than if I used a 2.5x multiplier and had the RAM at DDR875. Not to mention I don't have to worry about overclocking the RAM.
That's just a gentle starting-point. I would like to get the FSB over 400 eventually, but that will require some increases in voltage. Therefore I want to fit something other than the stock CPU cooler and maybe think about beefing up the Northbridge cooling, to cope with the extra heat.
In essence, you should think about dropping the multiplier on your RAM to x2 and upping the FSB. Try to target something around the 300-333 FSB to begin with, then see if you can get a little higher toward 350 - keeping a weather eye on temperature of course. Worry about pushing the RAM over 800MHz last - you've got to find settings where the FSB and the CPU are stable first, trying to overclock your RAM at the same time just adds something else that can go wrong. Once you've found an FSB and CPU speed you are happy with, and have Orthos-checked it, then try seeing how much extra you can get out of your RAM. But be aware that switching from settings where you have synced RAM at, say, DDR750ish and async at DDR850 will actually result in your system performing slower overall.
I've already ramped up the VDIMM by +0.2v as the stock seemed a bit low on the mobo.
