I've never really had any issues mixing and matching memory.
The only problems I ever remember having was mixing memory
within a memory bank. (IE. 4 in 30 pin or 2 in 72 pin.) Since
all DIMM's these days have their own memory bank, it's a
non-issue now. 🙂
If I decided to overclock, I wouldn't dare mix memory.
I'd go all CAS2 Micron, Infineon, Samsung or Mushkin.
For the longest time I ran 1 stick of Generic 128 MB
and 2 Generic Sticks of 64 MB on my Gigabyte GA-5AA
motherboard and had a wierd issue where when you first
turn on the machine, the machine would only count up
to the first 128+64 and would ignore the second 64 MB
stick. Hitting the reset button during POST would
always fix it. I just attributed it to a wierd quirk
in the BIOS and lived with it.