Not the RAM. I was referring to overclocking the CPU. For the memory, I would REALLY recommend that you are probably MUCH better off if you ditch the oddball 4GB memory module and just use the matched set of DIMMs, and then ENABLE the XMP profile for that memory kit in the BIOS. That will not be affected by updating and in fact, memory compatibility will probably be improved. You are much better off that way because you will have 4GB more memory than you currently have, plus you'll have memory that is running in dual channel AND it will be running at 2666mhz.
If you use that oddball memory module you MIGHT lose the dual channel operation, and you CERTAINLY will be running ALL of your memory at only 2133mhz.
IF that oddball stick was also 2666mhz then it would be worth trying to see if your board supports FLEX mode, so that you could run the two sticks in dual channel while that stick runs in single channel, because dual channel doubles the memory bandwidth and offer a nice bump in performance, but since your old stick is considerably slower it is not worth the attempt because it will negate any dual channel benefits by dropping the clock speed across the board.
You can try to add another set of sticks later if you find you need more than 8GB, OR, better would be to simply get a 2 x8GB kit later if you find this is not enough memory.