Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can be very sensitive to this.
This is more difficult when 4 sticks are involved.
That is why ram vendors will NOT support ram that is not bought in one kit.
It is safer to get what you need in one kit.
But, it is a bit more expensive because of the added matching of all the sticks to insure compatibility.
You want documented ram compatibility. If you should ever have a problem, you want supported ram.
Otherwise, you risk a finger pointing battle between the ram and motherboard support sites, claiming "not my problem".
One place to check is your motherboards web site.
Look for the ram QVL list. It lists all of the ram kits that have been tested with that particular motherboard.
Sometimes the QVL list is not updated after the motherboard is released.
For more current info, go to a ram vendor's web site and access their ram selection configurator.
Enter your motherboard, and you will get a list of compatible ram kits.
While today's motherboards are more tolerant of different ram, it makes sense to buy ram that is known to work and is supported.
My suggestion is to buy a 2 x 8gb kit and keep your old ram as a spare.
And... vengeance is a brand that you pay extra for. Tall heat spreaders are for looks and have no useful value except to record seeking ram overclockers.