If the ram works, you are good.
More ram will usually trump faster ram.
With some motherboards, you will still get dual channel operation with the original ram across two channels, and the odd capacity will be in single channel mode.
But... and a big but.
Adding ram is not 100% guaranteed to work.
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, particularly AMD can be very sensitive to this.
This is more difficult when more sticks are involved.
That is why ram vendors will NOT support ram that is not bought in one kit.
Best to go to a ram web site like Kingston and access their ram upgrade app.
Enter the make/model of your pc and see if they have a list of supported upgrades.
Otherwise, what is your plan "B" if what you buy does not work??
You may end up having to replace your 2 x 2gb ram with a 2 x 4gb kit.
In fact, I would buy a supported 2 x 4gb upgrade with specs the same as your existing ram.
cpu-z will tell you what you have now.
You will have a guaranteed 8gb. Then try adding your 2 x 2gb.
If it works, good, you have 12gb.
If not, keep the old ram as a spare or sell it.