Slowest DDR memory needed for fastest speed? R51 for example

[eyestrain]

What's the slowest DDR memory I can put in an R51 (400mhz FSB), without slowing it down, compared to using the fastest memory that exists? How is the figure calculated?

Now with DDR2 computers, I believe you ideally would just try to use the same speed memory as your FSB or higher. (True??)

But with DDR: I've doubts an R51 can make use of 400mhz memory, implying 3200MB/sec. On recent tests with Memtest86+ I've seen that R51 and T42p memory access is quite slow, below 1000MB/sec. X40 reads its DDR faster, but still nowhere near 3200MB/sec.

I understand that it is usually fine to use faster DDR memory than needed. I want to avoid using too slow memory. Want to see which computers my PC-2100 stick can go in, without ever slowing it down at all. And 1600 if I have any.

[dr_st]

It all depends on the RAM speed supported by the chipset, and there is no relation whatsoever to the FSB speed, even though it might seem such.

R51/T42 support DDR-333 as maximum speed. You can use DDR-400 but it will downclock to DDR-333 speeds.

PC2100 (DDR-266) will slow the systems down. But you can use them in T30/T40/R32/R40/R50e without slowing down, since those machines' memory controller are limited to DDR-266.

[eyestrain]

Thanks.