I am going to upgrade my 8gb desktop pc.
I have 2 free slots and 2 slots with 2x4gb 2400Mhz.
I will buy 2x8gb with 3600 MHz.
Should I put them together and have 24gb at 2400Mhz or should I remove the 2x4 in favor of the 3600Mhz.
I'm asking because I read that when you have 2 different ram speeds it will default to the lower one.
Edit: it's for gaming and I have a Ryzen 3 1200 with a b350m as a motherboard
I highly doubt that 350 board will support 3600 mhz. More likely it only supports 3,100 but will likely only be stable at around 2,900mhz.
Check your board specs and (this will help a lot) look up on the boards website the list of specific ram and speeds it has been tested to work on, and stick to buying something on that list. It will save you headaches.
Also, stick to just using 2 ram slots. Even if you have 4 of the same ram sticks, boards still actually run better using only two of them (two 8GB sticks run better than four 4GB sticks).
On a system that old, you definitely don't need over 16GB. Ram won't be your bottleneck.
look up on the boards website the list of specific ram and speeds it has been tested to work on, and stick to buying something on that list.
This is actually not entirely true. I went with a set on the QVL for my mobo and wound up with unstable RAM out of the box; I had to manually tune it. The list is really just a guiding light rather than a promise of viability, and you should run stability tests every time you drop in new RAM.
You should do a stability test, but that list is supposed to be confirmed tested viability. If you were unstable out of the box there's a list of reasons for it, and mobo incompatibility isn't at the top. It could be anything from the ram sticks themselves being faulty, to needing to be up to a certain firmware version on the mobo, to needing to change to a different compatibility/tuning mode to needing to manually change the timings (the qvl shows what timings the ram runs correctly at, it's seldom "plug and play"), to lastly, the ram tested for qvl may have been made in Japan, while the sticks you bought, despite being the same model number/brand were made from a factory in Taiwan or Korea.
Bottom line, your ram not working out the box does not mean the mobo qvl is incorrect.
It was Hynix C-tier ICs with an XMP set at 4000 M/T, C18. It was subpar binning, tbh, but I don't entirely blame the QVL checkers. It appeared stable on first glance, and it passed most tests (e.g. TM5, OCCT), but it failed every time on prime95 around 15min in.
It's fine, though. I ended up learning lots about my RAM, and now I've got it stable at 3666 C16.
I just like to point out that the QVL isn't a guarantee of quality, and it's not the final word on what will work, especially since those lists can sometimes be sparse.