Help with memory frequency

Hi,
I want to ask something about RAM.

Is it ok to use a lower frequency RAM than the one specified in the motherboard manual?

I have a Gigabyte 8I945PL-G, it says it supports:
4 DDR II DIMM memory slots (supports up to 2GB memory)
Supports 1.8V DDR II DIMM
Supports dual channel DDR II 533(Note 2)/400 DIMM

And from two years ago up to now I'm using 2 DDR2 RAM 256MB with the frequency of 266Mhz. Sometimes some problem with the RAM occurs.

And now I want to expand the memory,
What memory should I buy?
Should I buy the ones with the same frequency from the older one?
Or should I buy the one with the 400Mhz/533Mhz frequency?

Thanks

Presumably your current memory "with the frequency of 266MHz" is DDR2-533 memory. It sounds like you have a very old motherboard, which has some limitations on supporting DDR2-533, but will support DDR2-400 RAM just fine. What does "Note 2" in your manual say?
In any case, DDR2-400 memory is no longer sold, and DDR2-533 may also not be available. I would just buy some DDR2-667 RAM rated at 1.8V (not any higher!) and try that. Hopefully, it should automatically run at the slower speed that your system allows.