Mixing 6400 with 5400 ...

My system has 1GB of TWIN2X1048-5400C4PRO.
It is not overclocked and the frequency is set to Auto in BIOS setup (so it might be running at 667MHz ? how can I check it ?).
I plan to upgrade RAM to 3GB.
A matching 2GB of TWIN2X2048-5400C4PRO appears to be at the same price as TWIN2X2048-6400C4PRO.
Therefore I prefer to get the 6400 version (so that in the next system I could stick to 6400).
Can I "mix" 6400 with the existing 5400 ?
I think I can but I need to get the frequency set "correctly".
Would I need to change Auto setting in BIOS to a specific value so that both banks are accessed at the same pre-set speed ?
I may consider overclocking 5400 a bit to 711MHz. Clearly 6400 would cope with it without any problems. Would this cause any imbalance ? Shall I stick to nominal 667MHz for 5400 ?

Thanks and Regards

Robert

CPU-Z is a good tool to check your CPU speed, memory timings etc. It's free.

As far as your plan for 3gb, the only thing to do really is to buy and test. The 2 different types of memory may or may not run well together but you cannot know until you install them and test it. If they do not run well together, I would suggest simply using 2 x 1gb and finding another use for the PC5400.

Also, I would suggest setting your memory timings manaully to 4,4,4,12 or 5,5,5,15 if you get instability. Also, set memory voltage to 2.1v.