6-Layer Stackup for PCI express design
So long as you pay attention to trace impedance, signal return paths, and all of the other usual signal integrity things then you can really do anything with the stackup.  Of course, some stackups make it easier to do...

I have done several PCIe designs and what I do is this:


Signal
Ground Plane
Signal
Signal
Power Plane
Signal


The spacing between all layers, except between 3-4, is small.  Maybe 3 to 10 mils (not mm).  The reason for this is to give the signal layers a low trace impedance with respect to the planes.  This also means that the space between layers 3 and 4 is large-- large enough to make your total PCB thickness correct.  You will have to do the math to figure out what exactly works for you-- balancing trace width with trace impedance and stackup height.