I've got a "server" I built last year with 4 RAM slots that takes 8GB per slot. I say server in quotes because its not really enterprise grade, its just an mATX board that takes 32GB of RAM. Right now I've got 8GB in it, and that's not enough to run Proxmox VMs very effectively. (If anyone is wondering I'm on a GA-78LMT-USB3 with an FX-6300) I want to throw a 16GB kit in there for 24GB total, but there are so many different 16GB kits...
I need it to be 1600mhz, and reasonably priced. I don't need any ridiculous 2400mhz Dominator from Corsair, I just need something thatll fit in the slots without generating too much heat, and give me some more RAM to spread amongst my containers and VMs.
If you are going to put another kit of ram in it is HIGHLY recommended to buy the exact same model/timings. Mixing ram, especially for a server, is a bad idea. You want to find the 16GB version of the ram you already have and buy that.
Does that only matter for dual channel? I knew i t was important for dual channel but I don't think I need dual channel that badly, correct me if I'm wrong.
i've got pretty much the exact same issue as the OP, aside from that my server is a few years older, and the same memory is most likely no longer available.
how exactly the same does it have to be to be on the safe side?
Well, if the ram sticks you have now aren't sold anymore, find one with the same timings and frequency. If you still can't find one, then get one with tighter timings and higher clock speed. Then in the bios, setup to run at the slowest timings and frequency.
Dual channel or not, you want the same kits with the same timing/speed/voltage just so as not to cause instability issues. Mixing ram can work dont get me wrong, but ram is one of the most obnoxious problems to diagnose especially if it works 90% of the time but hiccups 10% of the time causing a crash.
I would search the model number of the ram you have and buy that. If it's ddr3 i can almost assure you its still out there somewhere. DDR2 maybe harder to find, so if you have to mix kits get something that is exactly the same timing/speed/voltage/etc OR better. If you get better spec than what you have already, you can usually downclock it to meet the other kit.