Normally, if new RAM is installed which is faster than the speed set in the BIOS, it will just run at the slower speed.
Have you reinstalled the old RAM just to make sure thats where the problem lies?
Are both modules correctly seated?
Try installing one module at a time, RAM is normally reliable these days, but a fault is still possible, even if the motherboard is Dual Chanel it should work with only one module fitted.
If the BIOS can't be altered to match the new RAM speed, perhaps the motherboard need a BIOS upgrade? Information about this should be available on the manufacturer's website, (once you are back in action!)
That's all I can think of at the moment, hope it helps.
Gibbo.