4gb (4 memory sticks) at 400Mhz on socket 939 MB -- JEDEC specifications ...

[hahn at physics.mcmaster.ca]

> At DDR400, that's 1 DIMM per channel, so maximum 2 DIMMs.
> At DDR333, that's 2 DIMMs per channel, so maximum 4 DIMMs.
>
> Expect any unstable system if you do otherwise.

I believe this ignores another relevant factor: how many chip-selects 
the dimm uses.  my understanding is that if a dimm is "double-sided", 
it means that it uses 2 CS's.  table 46 on p 182 of AMD's bios guide
(doc 26094) appears to allow up to 6 ranks for ddr400/2T operation.
afaikt, a double-rank dimm is AKA double-sided.  unfortunatley AMD also 
adds some weaselish text about how MB vendors should validate, etc...