Q: Can I use a 667 ddr2 memory stick instead of 800 ddr2?

Which model iMac do you have? You really need to make sure you use the correct memory for your specific system. You can use this article to locate your model and then find out what the memory requirements are.

http://support.apple.com/kb/HT1423

It sounds like you have an iMac Early 2008 model which does require the 800 MHz DIMM's. I wouldn't chance putting a slower DIMM in your system. IF it were the other way around I would have a different response.

Not totally impossible that some 667 RAM could support higher speeds, but not at all likely it would, & the RAM must be able to perform at 800 & have certain info programmed into it.

http://eshop.macsales.com/shop/memory/iMac/Intel_Core_2_Duo_PC2-6400

Good idea, as I find the 6 GB limit of RAM on yours & Mine not really enough for today's bloated Software.

You can not mix ram speeds, it would make your mac unstable. If you want to use 667Mhz DDR2 RAM you can, but both modules would need to be that speed.

As you you only have 1Gb, you could upgrade to 6Gb DDR2 667Mhz (1x4Gb and 1x2Gb) if thats the only ram you can get a hold off, but 800mhz ram would be better. 6Gb is more than enough for any normal user.

If you have a Mid-2008 model your graphics card (GeForce 8800) will run diablo 3. If it has the lower Radeon 2600HD Pro, like mine, then maybe not... but you never know xD.