I agree with airspirt. It sounds like your PCI bus is holding you up.
Try removing all your non-essental PCI devices from your bus and see how high you can go. Another thing to keep in mind is that your HDDs are on your PCI bus and if they cant handle the higher BUS speeds, then they will probably be the problem by corrupting data.
The 5x divider on the MB (if you have one) would then be the solution. Otherwise, your pretty much screwed, sorry.