m4400 Ram troubles (not liking my 667mhz dimms)

Ok, I was under the impression that 800mhz mobos were backward compatible with 667mhz dimms, so I swapped out my 2 stock 512MB dimms @ 800mhz for 2 2GB 667mhz dimms I already had laying around. But the system starting freezing and after swapping out the mobo for a new one I finally discovered that if I put the original sticks back in the problem went away. The 2GB dimms are not the problem as I've run diagnostics on both my HP (which never had any problems the 2years I've used those dimms) and the Dell which both say the dimms are fine. I've also tried using 2 other dimms (1x1GB and 1x2GB, both @667mhz) with the same results. So the only conclusion I can draw is that the problem is because of the fact that they are 667mhz?

Was I incorrect in using 667? If yes, I'm gonna feel like a complete idiot LOL but i guess it was bound to happen again sooner or later haha. I just really dont want to spend $50 on 4GB. Was trying to hold out for 4GB dimms when they're cheaper...

 

I have been running 8 GB of 667mhz RAM on my M4400 for several months now without any issues... I don't think the M4400 has any problems with 667mhz. I am running 64 bit as well and I was running Vista 64 prior to installing the Win7 RC.