I'm not sure what you mean by DDR mode, I'm assuming you're talking about dual-channel performance, if so then there really isn't that much difference anyway, I need more than 8GB of RAM, even now I have 2 counts of chrome open with about 20 tabs in each and my RAM is just giving me the finger
I want to open photoshop and edit and have a demanding game like arma 2 open in the background
I would like 16GB seems like a nice number, sure I'll probably never reach it, but i'll never reach my 550w limit in my PSU, I still want at least 500watts lol
It will not run in DDR mode which will decrease performance, and you won't use more than 8GB so 12/16 is wasted money. You'd be better of to get a matching 4GB stick and be done.
DDR RAM will always work in DDR 'mode' but asymmetrical DIMM configurations between channels will make it operate in interleaved rather than dual-channel mode, which may limit the system to about half its potential RAM bandwidth.
Interleaved (asymmetric DIMM configuration between channels) is still technically dual-channel but cannot match dual-channel bandwidth unless access patterns spread evenly between channels, which is unlikely to happen unless software is written specifically to take advantage of that uncommon and generally not recommended setup.
Personally I have 12GB of ram in my build, despite realizing how unconventional it is it has worked fine for me for the life of my system. I have the full available 12gig capacity despite never using over 40% at once. My mobo has two pairs of dual channel slots, I just have 2X 2GB in one dual channel section and 2X 4GB in the other so it is balanced out and works fine. However, I could see why those above might think it's not good to imbalance a single dual channel setup. Putting 1 8GB and one 4 GB in a single dual channel section of your mobo will likely cause problems such as described by others above. If your motherboard has 4 slots, if you place your 4 and 8 sticks in every other slot it might be okay, but I think it is probably better to just run 8GB in dual with two 4gb sticks. Unless you do hardcore/high quality video editing and ripping you will likely see no benefit to 16GB over 8GB. Gaming configs with 16GB certainly have no benefit over those with 8GB unless you're one of those insane people running triple monitor setups with over 6000 pixels across your display and don;t have an adequate graphics card for that. For photo editing, honestly just having a graphics card that runs Arma to a good standard in your system will do the trick for all your 2D editing/scaling/altering needs. So I would imagine you're fine with what you have in that department.
And for the record, I have yet to see a well built custom gaming rig for which the number of chrome tabs running simultaneously even factors into things