Making use of DSR isn't too terribly complicated. You need a Maxwell-based GeForce video card, like the GTX 970 or 980, and a relatively low-res display. The DSR options aren't available with a GTX 980 connected to a 4K monitor, but they do show up at 2560x1440 and below.
To find them, navigate to the "3D settings" section of the Nvidia control panel.
You can then choose which DSR modes will be exposed to games by checking a series of options in a dialog box. I turned 'em all on, so I could choose from a whole host of DSR resolutions up to 3840x2400—four times the native res of my 1920x1200 display.
The "smoothness" slider lets you control the sharpness or softness of DSR's downscaling filter. The default of 33% works well, in my opinion, but it's possible to drop the "smoothness" to 0%, which will be very sharp and noisy, or to 100%, which is pretty smeary. I have some examples of what happens when you tweak this slider coming up.
After you've enabled DSR in the control panel, it will "fool" your games by offering them resolutions higher than the display's native capacity. The example above comes from Guild Wars 2. I'm able select from seven different resolutions that are higher than my display's native size.
First-world DSR problems
Once you're using a high-res DSR mode, you'll be faced with some of the same problems that owners of 4K monitors often see.
Not all games are well-equipped to handle really high-PPI (or effective PPI) displays. Guild Wars 2 has a number of different interface size options, but even the largest one results in some pretty tiny icons and a minuscule mini-map in the example above. The situation is improving as newer games adapt to a high-PPI world, but remember that DSR is most likely to be a live option with older games, those with less demanding GPU requirements than brand-new, top-shelf titles. These issues vary with the type of game and the sort of interface it uses, though. I suspect quite a few older games will work just fine.
Then again, not every new game is demanding enough to make DSR impractical. For the sake of testing, I eliminated all doubt on this front by doubling up on GTX 980 cards.
Yes, that's right, I used dual GTX 980s in SLI to drive a 1920x1200 display.
That may sound like overkill, but remember that the cards are rendering internally at 3840x2400—and 4K resolutions are still challenging for any single-GPU config. With two GTX 980 cards, Unreal Engine 3 titles like Thief are not a problem.
Other games, like Guild Wars 2, Tomb Raider, and Battlefield 4, need every ounce of power that both of these GM204 GPUs could muster and sometimes still want more.
Notice above that I've included results from a native 4K display (at 3840x2160). As you can see, using DSR at 3840x2400 and scaling down to our 1920x1200 display for output is slightly slower than rendering natively at 4K.