Dual channel help

Okay, so I have a machine that has 2 4GB DDR3 sticks in it. I want to eventually go to 2 4GB sticks and 2 8GB sticks for a total of 24GB, but I want to be able to do dual channel. If I buy a single 8GB stick by itself now, and then in a few months buy another one of those exact sticks of RAM, will it work in dual channel with the first 8GB stick? Or do I have to buy a 16GB kit with 2 8GB sticks in it to get dual channel?

Yes it has to be a exact match even off by i digit in the part number and you could be screwed .... and the 8GB sticks should match the Cas Latency of the original 4GB sticks  ... just put the big ones in the primary slots and move the 4GB to secondary

Okay thanks. Just curious, how much of a speed difference will dual channel make for a Proxmox server? If its not going to make a big difference for some average simple 1600mhz ram, I probably won't even worry with it.

Always buy factory sets of 2,3,4. They are matched and tested. 

Do you know if it will make a difference to not have dual channel on a virtualization server? This isn't for gaming or anything its just Proxmox so it may not even matter