How would a motherboard/chipset limit Memory speeds on an A64?

[KillaKilla]

Seeing as the memory controller is on the CPU, and, I assume, the memory never directly interacts with the chipset or any other components. Why would A64 chipsets/motherboards have any memory specification, beyond:
?Maximum electron frequency over the distance between the socket and the memory
?DDR, DDR2, DDR3, SDRAM, etc
Wouldn't all memory dealings happen between the CPU and RAM? IE if a 939 A64 that supported memory up to DDR-550 (275Mhz) was plugged into a motherboard that only went up to DDR400 (200mhz) would it's RAM be able to go at DDR550 (assuming that the CMOS didn't block it)?

Sorry if I'm not being clear enough about the idea...

 

[Matthias99]

The motherboard no longer has the memory controller, but still contains all the physical interconnections between the CPU and RAM. If the motherboard's components are poorly shielded, or it can't provide clean enough power to the DIMMs, that could easily limit your speed.

But, yes, barring any sort of physical-layer or BIOS limitations, any speed DDR RAM should work on any A64 motherboard.