Maxing memory speed, GB or MHz more important?

Hi,
I have an Intel q965gf motherboard with 4 DIMMs which will support 8GB of memory at a max of 667 MHz or 4 GB of memory at 800 MHz. Which of the above would be best to do for gaming performance?
In the specs for the board ( under supported memory configurations), it says that the largest usable DIMM for DDR2 at 667Mhz is 2 GB, thus 4 DIMMs x 4 = 8 GB. It also says that the largest usable DIMM for 800 MHz is only 1 GB, thus 4 x 1 GB = 4 GB max for 800. What would happen if I put 2 x 2GB at 800 Mhz modules into the first DIMM of each channel (ie. do I have to buy 4 x 1 GB modules of 800 MHz to get 4GB or could i install 2 x 2 GB of 800 MHz?)
Thanks in advance.
Thanks