It should work just fine, boards are typically rated "up to" the numbers stated in the specs.
If you can find some at a decently lower price than DDR2-667 or DDR2-800 then go for it. Although, there's all kinds of deal/sales on right now with DDR2-800, might be worth shopping around a bit.
What processor are you looking at? Q6600? That will work fine with DDR2-533 because the Q6600 is 1066fsb (1:1)
However, if you get any of the 6X50 chips, they are 1333fsb and would prefer some DDR2-667 (to run at 1:1)