Why my memory cards Mhz's are low?

Hi all, I need some advice how to get maximum out from my desktops memory.

I had 4GB of RAM installed already in my desktop, when I decided to add another 4GB.
The things is I couldnt find the same memory and finally went with
another company's memeory card.

Problem is, that when I checked with "CPU-Z" program, the status of my memory
card, it showed me 533Mhz and when added additional 4GB, the Mhz went down
to 399Mhz

Firstly, I already had CORSAIR XMS2 (CM2X2048-8500C5) DDR2 memory, what
should be 1066Mhz and added KINGSTON 2-KIT KHX8500D2K2/4G, what is
also 1066Mhz.

My question is, how come the Mhz is down to 533Mhz and when
added additional 4GB, down to 399Mhz?

Thank You guys and girls, for Your time!

I haven't found anything in the manual that states this. but some motherboards clock the RAM down if all 4 DIMM slots are populated.

If you set the RAM frequency, voltage, and timings manually in the BIOS, you may be able to get it work reliably at the rated frequency.

Thank You for Your reply.

I have to do it in BIOS then - is there any good tutorial or guide You could point out?

Is it safe and ok to manually increase those settings in BIOS - I mean, doesnt it shorten
lifetime of the memory card or corrupt it anyways?

And finally - can both memory cards (kingston and corsair) be used at the same time and
also increased the Mhz settings manually from BIOS to both?

Maybe stupid question, but better be sure than sorry later :)