I have 2x4GB of Corsair Vengeance in my system and I was wondering what would happen if I add 2x8GB more. That would give me 24GB of RAM total. Would it cause any performance issues?
It is not recommended. Use only matched pairs. It will downclock to the least common settings among other issues. Although I have done it in the past without issues. No company will tell you it is okay. Might have problems and you might not.
On the topic, I have 2x8GB sticks of G.SKILL Trident X @ 2400. If I bought another set of this, the very same thing, could I plug that in and know that it will work?
All I will say is they still wouldn't be a MATCHED set.
The problem with memory is that it goes through many revisions within the same product code. For example a the Gskill 2400Mhz 9-9-9-24 (or whatever it is) of revision 1 and revision 2 may have issues with eachother.
Although memory has two modes of operation, ganged and unganged. For ganged operation all the memory needs to be identical, this is where the real benefit of dual/tri/quad channel memory comes in. Unganged (albiet slower) allows you to run unmatched pairs, but it will only run at the speeds of the slower memory. (unless you overclock the memory).