I read both the user manual and the hardware maintenance manual(HMM) and neither mentions the need to pair the memory or placing them in certain slots depending on size as the W510 article does. I asked around and different people tell me different things.
Also, I wonder if the the W520 can actually take advantage of the faster 1600 memory (DDR3 1600) or does it merely 'support' it.
My ordered memory configuration is 1-4GB (1333) stick. Machine with processor i7-2860QM.
The RAM slots of W520 are identical to the W510's. Thus, the existing knowledge base article about W510 memory optimization should work for W520.
Considering you have QM processor, your can use the faster RAM (DDR3 1600) The W520 with the quad core processors all support the higher speed of the DDR3 1600 RAM. If you want your system to use dual channels, then pair the DIMMS by following the knowledge base article W510 memory optimization
There have been various posts about the speed increase. In my opinion, all the memory will run at the lowest speed if you mix them with different speeds.
There are also some additional considerations:
While upgrading or servicing the system, the 2 dimms are not put back in the correct slots, the system can take a very long time to boot up and will run with reduced performance.
Dual core CPU equipped models can only support two memory dimms. Some systems ship with dummy modules populating the slots that should not be used. If these are removed and additional memory added to these sockets, the system may not boot and may beep a 1-3-3-1 code.