OK I decided to read up myself. Here’s what I think I’ve found, at least regarding AMD chipsets:
The Front Side Bus that I’m used to exists no more. It has been replaced with two external buses, a memory bus and a hypertransport bus. The memory bus links RAM to CPU, the hypertransport bus links CPU to the rest of the peripherals.
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But all Athlon 64’s run off of a base clock of 200mhz. So you have a CPU multiplier and a memory divider. My CPU multiplier will be 3000/200=15. My DDR2 800 will give me an 800/2 = 400mhz memory bus, which gives me a memory divider of 3000/400=7.5. But! The memory divider rounds to the next highest integer value, which is 8, so my memory bus will only actually be running at 3000/8 = 375mhz or a double data rate of 750 mhz. :-\
So I may try to overclock to get the most from my memory. What if I use a CPU multiplier of 16?
200x16=3200mhz
3200/8=400
400 is my advertised ram speed, so if I can overclock my proc by 200mhz then I should be able to get the most out of my memory. That is assuming that my mobo offers a multiplier of 16.
I could also try increasing my processor base clock from 200 to, say, 210.
210x15=3150
3150/8=394 or ddr2 = 788mhz Getting closer.
So I can play with my multipliers, dividers, and base clock to find the fastest stable combination I can. I’m guessing the mobo will automatically set me up for 3ghz proc, 375mhz memory bus, and 750mhz memory.
If I had wanted to spend more money (and had figured this out before today) and wanted the fastest stuff that’s basically sure to be stable I could have bought DDR2 1066, which is the fastest memory my motherboard supports, and used a memory divider of 3000/533=5.6 => 6. That would have given me a 3000mhz processor and 3000/6=500*2= 1000 mhz memory.
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So now the hypertransport bus. It’s also based off of the CPU clock. Keep this in mind when fiddling with your base clock when trying to OC.
My hypertransport bus will be limited by the processor to 2000mhz even though the board can handle up to 2600mhz. So it has a hypertransport multiplier of 200x10 = 2000mhz.
So I can only transfer 8 GB/S instead of 10.4 GB/S. (Transfer rate = number of bits, AMD uses 16 bit links x clock x number of data per clock cycle / 8 bits per byte, so 16 * 2000 * 2 / 8 = 8000)
My peripherals base their transport speed off of the hypertransport clock and have their own dividers. PCIx runs at 100mhz so 2000/20=100, sata ports share this clock apparently. Some motherboards let you adjust these dividers, some don’t.
So if I fiddle with the base clock to overclock, EVERYTHING has to be able to run faster. One change affects everything in the system.
In other words, when I put my rig together tonight I will just accept the 750mhz DDR2 speed which will keep everything else running at OEM speeds. Once I’m convinced that everything is nice and stable then I may try fiddling with OC settings.