You'll see there is some DDR2-800 RAM on the qualified vendors list.
I'll give you that it would be better if both board manufacturers and RAM suppliers made it clearer what speeds will work, and at what speeds they will work at.
You shouls also be aware that there are more than one version of the motherboard, and many BIOS versions - I can't see it made clear exactly which supports what in either case.
I would err on the cautious side - but exactly what is your issue? Are you upgrading? - in which caase you might need to buy RAM which matches what you have - if you are totally replacing, then buy in matching pairs - for instance.
Mixing RAM speeds is a very poor idea. You've got a dual-channel motherboard. To get the speed advantages of dual channel RAM, the RAM modules must be fitted in matching pairs in the correct sockets - I see your board has 4 sockets.
What do you have in them right now, and where would you like to get to?
PS what OS are you using - 32-bit Windows will only give you around 3 to 3.25 GB available RAM, no matter how much you fit (max for your board is 4GB I think.)