Can you run DDR2 in a DDR3 capable board?

I have seen a few P35 boards which can run P35, at the moment I dont want to buy DDR3 as its still expensive but if I but a P5K3 Asus board which can run DDR3 is it able to run DDR2 (I thought the sticks are keyed differently so DDR2's wont fit in DDR3 sockets?)

Reason I posted this is that I saw a Gigabyte board which is capable of running both, from what I understand P35 supports DDR3 but depends on the mobo maker what they select?

^Agreed. Also if it has both some motherboards will let you run only one type, if you use DDR2 you won't be able to use DDR3 at the same time.

thanks guys, I guess these Gigabyte boards have both types of slot. TBH from what I've seen in reviews etc not sure DDR3 is worth the extra cash anyway