Go to the DCS site. Go to E-shop -> Modules. Each module has a minimum, recommended and VR set of specs. Your system is a bit "dated", but meets the min spec. Depending on your motherboard, you may or may not have the ability to add a Solid State Drive. The reason for this recommendation is the speed of access, compared to a traditional magnetic disk disk. DCS runs better from a dedicated SSD. You do need the 120 GB of space, though! I've got 400 GB on the dedicated DCS drive, with only a few utilities. The majority is taken up by the base game, modules, terrains, and campaigns. The more complex aircraft such as F-16 or -18 may or may not run well on your machine.
Having said that I ran DCS stable version with an i7-3770K, 16GB RAM, magnetic disk drive, GTX 770 SLI (similar to your GTX 970) with few stutters and not bad graphics, F-18C and F-16. Super Carrier had poor frame rates, though.
If you have the Power Supply capacity and CPU cooling, check if you can overclock the CPU. The game uses mostly a single thread, so overclocking does help.
Internet speed will set how fast your downloads (and updates!) are, and only affect Multi-player. If you've been here before, then you probably have some of the modules. The game engine will need updating to be able to run some of the newer modules (check on the site) and pretty well all of the current campaigns. Updates are pretty massive, so be patient. DCS Updater GUI may prove useful if you decide not to update to the current stable version.
Remember to have fun!!