Would it be bad to mix ram?

All I’m talking about is combining 4GB of Corsair 8500C5D with 4GB of 8500C5, I already have the C5D

Bad part about mixing ram is their timings/voltages. If the ram you're adding has the same timings/voltages, then it should work. However be advised that populating all the ramslots on the board will hinder your overclock. The more ram installed, the harder the memcontroller on the board will have to work. You may have to raise the NB voltage which means hotter NB temps.

There is no garantee that you will achieve the same overclock you have now once you add more ram.