Siaf,
The bios will automatically detect that there is pc2700 ram installed and will clock down the memory bus speed to 166 mhz for all installed memory. For the last several years, pretty much all motherboards have been able to run the cpu and memory asychronously so at default settings for an athlon64, the cpu will be at 200 mhz and the memory will be the rated speed of the memory, up to 200 mhz.
The memory bus hasn't been locked to the cpu bus for a couple of years for most chipsets. Even the pci and agp clocks are now able to run at speeds independent from the FSB, and many can be locked at any speed you want regardless of the FSB.
If you put only pc3200 into an athlon64 mobo, it will detect and run at 200mhz (ddr400). If you add in a stick of pc2700, it will run at 166mhz. But if you put in PC4000, it will generally top out at 200mhz by default due to that being the max officially supported memory speed for the athlon 64 and all chipsets. You can go in and manually set the memory speed on many a64 motherboards up past 250mhz, but then you have to set the hypertransport bus multiplier down to keep the HTT speed below 1ghz.
So again the short answer is that adding a stick of pc2700 will result in the memory speed reducing to match the slower pc2700 speed. You can try to overclock the pc2700 back up to pc3200 speeds, but you'd probably have to increase memory timings if you wanted any chance at getting it to run reliably.
Plus more than 2 sticks of memory in an athlon64 mobo is not always guaranteed to work, and you might have to further reduce the memory speed by selecting 2T command rate just to get it to boot. If you don't need the memory, you're better off without it.
