Hello... OLD is 1333 and new is 1600... Plug it in and try it, you won't break any thing... It just might not boot... Depends on the BIO's if the 8gb and 4gb will be used/work together... 12 gb IS better than 8gb.
My understanding is that the ram will run at the speed of the slowest module, so you'll have 12GB running at 1333 instead of 8GB running at 1600.
That said, if you aren't using the 8GB (heavy multitasking or intense processing) fully, you probably won't benefit from the extra 4GB. Task Manager or Resource Monitor can show how much of your memory you are using, assuming you are on Windows.
Also try not to touch the gold contacts on parts, skin oils will tarnish the contacts and result in poor connections. If they get discolored, you can use an alcohol swab to clean them, and let it dry.
You could also use a soft pencil eraser, but do it away from the computer so the crumbs don't fall in. (Also be gentle, there is a bit of abrasive in erasers, and the contacts are very thin.)
Hello... Does Gold tarnish? only the "Gold colored necklaces" that you buy on street corners... Pretty stable element... and that is why it is used in computer parts... Oils and dirt can cover it... Chemistry 101?
So is it better to run 8GB at a faster speed or 12GB at a slower speed? Technology is confusing =/
12 GB slightly slower is better than 8GB slightly faster, in my opinion. RAM speed is usually barely noticeable (i'm reading 1-3 fps from 1600 to 2133. As I said before, though, if you are playing at lower resolutions and not multitasking heavily, you are probably not using the 8GB you have, to be honest.
An aside, if you have integrated graphics (which often relies on system memory), memory timing can become a more important issue, though I still don't think 1333 vs 1600 is going to matter much. The extra memory helps in that situation, too, since the IGP will need to reserve some for itself.