I've been running Windows Vista Business x64 for over a year now with 4GB of Kingston HyperX Ram (2GBx2) and it works smoothly, no memory errors, nothing. I just upgraded to Windows 7 x64 last night and suddenly everything went to snail speeds, especially IE. I popped out a ram stick out and it went back to perfectly smooth. Either stick works fine, just not together. I had used Windows 7 x64 RC before and that had worked fine on this same system.
What could have happened in the hour it took to move from Vista x64 to Windows 7?
That's where I would start too. Make sure all the mobo drivers are up-to-date. Even though it sounds more like a driver issue, it probably wouldn't hurt to make sure you have the latest update for you bios as well.