Off-hand, these are the differences I can think of between the 600/600E and 600X:
- 600/600E does not have the MiniPCI slot so there's no access to it from the bottom of the base; the 600X does have the MiniPCI hatch
So if you want/need/plan to put a 600X motherboard into a 600/600E base, then make sure you mount the MiniPCI card first

.
- 600/600E has a slightly different audio jack arrangement than the 600X
Which will probably prevent you from installing a 600X motherboard without some minor surgery to the base to make that area fit (and assuming that the mounting holes are the same).
- I
think a 600/600E lid assembly (with LCD cable) is transplantable onto a 600X base
I say "think" since I don't believe I swapped a full lid assembly. The HMMs say that the hinges and LCD cables are the same between the 600/600E and 600X, however the inverters have different FRUs.
There should also be some more information on this in the archives...