While it's technically possible to mine and game at the same time, it's not recommended. If you're playing something like Minecraft, then you might be able to get away with it, but it probably won't work with games that are graphics intensive, like Battlefield 1 for example.
YES!! You will definitely make money, even with your current setup. I believe the 390's hash at around 400 Sol/s, so in the first month you'll mine about 0.5 ZEC. Here is a good profitability calculator that factors in difficulty growth: http://www.mycryptobuddy.com/ZCashMiningCalculator.
The best advice I can give is: mine with what you already have, and buy ZEC directly with the money you wanted to spend on new mining hardware. You will earn less ZEC mining over time than you could by simply buying ZEC at the outset.
If you're building a gaming rig, then buy only what you would if mining wasn't profitable. When you're not gaming, you can set your gaming rig to mine, which will likely be enough to pay for itself. However, the equipment will never earn as much as simply buying the coin outright, because of the growth in network difficulty.