The two RAM kits are probably incompatible. Also if you have windows 7 home premium then you can't use more than 16GB of RAM. That isn't a reason for the computer not to work but if the computer did work and you have home premium only 16GB would be accessible.
So you want 18gb of ram in your Pc? I would just use the 12gb set , you can't possibly use more than that.
Why couldn't someone possibly use more than 12GB of RAM?