On K8s, the RAM Frequency is a ratio of the HTT Clock ("FSB") : RAM Divider/RAM Frequency Ratio. There are then two salient points we can glean from this: 1) K8 systems (indeed, all modern personal computer systems) are always running on a RAM divider (1:1 is still a valid ratio, after all); and 2) RAM speed is completely dependent on the reference clock (HTT Clock or FSB, on Intels or older systems).
Technically speaking, the only real compatibility issue you need to worry about when it comes to RAM has less to do with the frequency rating (DDR400; DDR500; etc.) and more to do with the pinouts/RAM type (DDR; DDR2; DDR3; etc.). If your system requires DDR, only 184-pin DDR will work on the board (the DIMM slot, and therefore the memory controller, dictates this). RAM frequency can always be altered to suit (i.e., overclocked or underclocked) in the BIOS through the use of the RAM Divider settings. Of course, if the system fails to POST, it will have everything to do with whether or not the RAM can run at the speed you're asking it to (especially if, through your settings, you're asking the DIMMs to run faster than they are capable).
Hope this helps!