Honestly, anything above 1333mhz isn't really going to do anything noticeable for standard stuff. You only need faster RAM if your going to be using very Ram demanding programs (Video editing, possibly?) or if your using an APU, in which case the faster the RAM the better.
For gaming, surfing the web, etc...1333mhz is fine, and will definitely be fully compatible with your board
EDIT: Ah, re-reading your post, I realize now you have mixed sizes of ram. To make sure you have a Dual channel, you'll need to make sure that the 2 sticks of the same type are in the same colored slots (Reference your manual to find out which 2 slots support dual channel), the non-matching sticks can then go in the other slots.
Can you provide more info - like the make of the mobo and CPU as well as the DRAM you have - is this a prebuilt which may well limit your DRAM options, but regardless, it not a good idea to mix DRAM - your best bet would be to buy a single set of the full amount you want
And also, just to confirm - should I definitely be removing both of the 2 modules that came with the computer? (8gb + 4gb)? Will it cause issues if I left the other 8gb in? I would love to have 24gb total (comp has a 32gb allowance). But if it'll cause potential issues then I'll just remove as you suggest.
Could go with either of those sets, would start by just trying them, if all Okay then try adding the other 8 GB stick, and if that will work then try adding the four GB stick - if you need help along the way give me a shout
This kit will do anything the Dominators will, except cost you that additional $50 bucks. Would have used one of your sites but I figured since you were open to more than one you might be open to this one. Same Speed, Same CL, Quality brand, $50 less.
EDIT: That set tradesman just posted is basically the exact same memory I posted just different heat spreader pick the one you like best and use it, will perform the same.