did u reinstall the Operating system after the upgrade, if not this could be a contribution to ur problem.
The Memory could be an issue because of the difference in speeds.
Is the memory set at 800 in the Bios as this could be O\C the 667 ram and causing instability,
what brand of PSU have u got, as 500 should be enough although depending on brand and quality, more power will be required with the 4870x2
These can all have factors in ur issues.