Here's the official system requirements:
https://www.frontierstore.net/planet-coaster-earlybird.html
I don't think it's a good idea to go out and randomly buy a graphics card especially from a non-manufacturer like ebay etc unless you've done some research to make sure your system is going to be compatible with it.
I invested aka bought, a NZ$2500 custom PC desktop system from a special NZ company who lets you customize your PC and THEY build it for you and run tests and inform you if any changes or additions etc are necessary. It did kind of break my budget but I'm happy with how well it performs. It has the almighty 6th gen Intel Core i7-6700k processor, Sapphire Nitro with back plate AMD RADEDON R9 390 8GB GPU, 16GB HyperX system RAM, a Kingston HyperX SSD with Windows 10 OS and my games on it, Wi-Fi ac, and it can play this game and my games in MAXED OUT settings in 1080p with good smooth frame rate. It can play the sims 4 in ultra everything settings in 1080p 60fps.
The AMD Radeon R9 390 is more better than the GTX 970 according to GPU boss. R9 390 also has a higher bandwidth of 384GB/s, higher bus 512-bit, and 8GB GDDR5. The GTX 970 has 4GB, but only 3.5GB of it is fast memory. The last 0.5GB is slow memory. Some peps may say you don't need 8GB. But 8GB or more helps for 4k. AMD and Sapphire imply the R9 390 is capable of 4k. And AMD recommend at least 16GB memory for 4k, which is what I have. Some peps may say that the R9 390 is not high end. When in fact recent sources and sources on the net, state that it is. It's not the best GPU, but it is more than medium core GPU. I have The Sims 4, cities skylines, planet coaster, and final fantasy XIII on it. I also have an HP 15' ab222tx laptop for everyday needs. I also game primarily on Xbox 360 and Xbox One. But I feel Frontier does a great job with this game I maze well enjoy the full flavour of this game by getting a system which can bring me that. And my system does. My system has overkill hardware XD. Not the best, but it is pretty good. And yes I can't stop bragging about it. I'm proud of my system. And I never use of 7GB of the 16GB of RAM my system has. The R9 R90 can handle all 2015 and 2016 games on maxed out settings in 1080p. I got Xbox 360 late 2010. Final Fantasy XIII was my first game on it and I loved it. So got the PC version off Steam for only NZ$18.99. It has higher resolution I've turned on x16 anti-aliasing and my system plays it comfortably. And since my system has an SSD it takes like 1 second to save and load.
Also the 2GB GPU will be your bottleneck because it only has 2GB graphics. So you may run into bottleneck if you play this game on medium or high settings. Good tools you can get for free on the net is MSI afterburner its free and displays fps and your hardware performance in game. If researching for GPUs and CPUs GPUBoss and CPUboss and notebookcheck I find good. tomshardware.com or something like that and gamingdebate otherwise google are good tools too.