So, I currently have 8G of RAM installed in my PC.  I recently got X-Plane and would like to add more RAM so I can make use of the HDMesh/OSM stuff that's out there.

 

If I just add 16G, would that work ok?  I would assume I'd want to at least match the timings as best I can?  Or, do they need to match exactly?  My current RAM has timings of 9-9-9-24 i believe.  If I get RAM that is 9-9-9-27, will that be good or should all those numbers match?  Oh, speed is 1600 on current so the new RAM I add of course would also be 1600.  Or, do I have to instead buy all new/matching RAM and discard the current RAM?

Unfortunately I can't give you a definitive answer, but I did read this article in which a couple people advised against using ram with different timings:

 

http://www.tomshardware.com/forum/289757-30-mixing-timings

 

Best of luck and I hope someone who knows more than me responds (haha)

Kevin, I would just sell the older ram and install a new matched set of 16GB. Use 2133 or 2400 ram, lower timings the better. If you choose to mix in the new ram with the old you would most likely have to run both sets at the lower value meaning even if you bought 2133 you would have to run it at 1600 to match the other ram. But 16GB is enough for most people, so just pull out the slower ram and enjoy the newer stuff.

Agreed with the above

 

I tried to add 8 (so I'd have 2 sets of matching pairs), but the recent set wasn't perfectly identical and the PC stopped booting correctly.

 

Argh

 

So yes, get 16GB if 8x2 if you can

 

 

But 16GB is enough for most people

 

So 16GB will be enough for HDMeshv3 and World2Xplane OSM stuff along with photo real and any enhanced airports that may exist.