My i7-2600k rig with a GA-Z68AP-D3 motherboard currently has 4x2GB 1333Mhz memory (cheap memory from a while back) and I have another computer that needs two of these sticks.
If I purchase 2 4GB sticks at 1600Mhz with 9-9-9 timings to replace the two RAM sticks I will use elsewhere, should I leave the other 2x2GB 1333mhz RAM in and run 12GB asynchronously or leave it at the 8GB 1600MHz?
I don't think that you can run memory in asynchronous frequencies. You'd probably have to run them all at the highest frequency supported by all modules. Mixing kits is also not recommended at least unless they are two of the same kit. Mixing kits can cause the computer to not work properly because sometimes, different kits just don't work together even if they seem like they should. I suggest that you remove the 2x2GB kit from that computer. You can put it in another computer, sell it, give it to a friend, or whatever else that you might want to do with it, but leaving it in your computer is probably a bad idea.