the actual clock rate of the memory and memory bus. HOWEVER because it is DDR SDRAM it reads/writes on both the leading and trialing edge of the clock, THUS 2 read/writes at 200 MHz = 400 MHz effectively. It is simply a terminology issue.
I doubt if your CPU supports Dual Channel, HOWEVER understand that Dual Channel IS NOT a memory characteristic. Dual Channel is a mobo, chipset, and CPU function the memory used is simply standard DDR memory. Just uses a pair of the same size and type.
If you run the memory bus up to 250 MHz you may fry something. The mobo is simply giving you a capability to overclock your memory bus. That is usually done in 1 MHz steps until the system becomes unstable and then back down a MHz or two. Lot more to it than that and you risk frying things.
I've just bought a new barebone ($100 TigerDirect):
Biostar M7NCG 400 nForce2 Socket A Motherboard
AMD Sempron 3300+ Socket A CPU
The mobo supports FSB of 400MHz (also supports dual channel). The RAM I got is PC3200 1GB (2x512MB) dual channel and is identified correctly by BIOS - it says Dual channel status. PC3200 is 400MHz. The processor speed and the RAM ammount in Windows is showing correctly too (2.2GB and 1GB or RAM).
However, in the BIOS settings it says that my FSB is only 200Mhz! It should be 400Mhz.. I tried to manually change that but FSB only goes to 250Mhz!? So, there's a % option.. So I tried 200Mhz x 200% = 400Mhz.. But the CL became 3.0 instead of 2.5 and the PC would not boot.. I pulled the CMOS battery and am ready to try again
How do I get my RAM to run at 400MHz speed?
As you can see I'm a bit new to 'Advanced' BIOS settings, so please explain this as you would to a ten year old..