4 DIMM dual channel question

[mcpigeon]

I'm planning on building a socket 939 setup soon. Probably using a motherboard like the ASUS A8V. It has slots for 4 DIMM's using 2 sets of matching pairs. I am wondering what happens if the 2 pairs are not matching. More specifically, I am thinking of buying a nice set of OCZ ddr500 for one set and I have the ability to get a large amount (possibly 2 Gigs) of very slow timing ddr400 in a second matching set. Assuming the new chipset doesn't have any problems filling up all the slots like some of the older socket 754 did, what should I expect? Is it possible to run one set at a faster timing than the other (or has it been in any other chipsets)?

[amd_man2005]

You can't run them at different timings and alot of times it will not work unless its identical memory