If you use both sets of ram, all of them will run at the lowest speed of 667 mhz.
Put the 667mhz ram in the first set of slots then the 800 mhz in the second set of slots.
Although your system (x32) does not fully use 4gb, it might use as much as 3.5gb, depending on the motherboard and CPU.
If you hear any beeps or there is a lag, it is better to just use the higher mhz (despite what others might say) as long as the motherboard ram specs allows it. Don't bother with overclocking or underclockng, you will just wreck the ram, leave as default unless you really understand the consequences.
If you want a more definitive answer, provide your CPU and motherboard model numbers, and manufactureer and specs.