To tell what is better is really easy in 90% percent of cases. The higher number is the better product. You can also type in product X benchmark, and find 100's of sites with the information you want compared with other like products. To run any source game, i.e. Garry's Mod, you can pretty much run them on the bottom of the line parts, while games like the Witcher 3, BF4, GTA V and other graphically intensive games will benefit greatly from a 970, 980, 280x, 290x. That doesn't mean that a 560ti or a 7850 can't run these games, but you will start compromising settings big time. So to give you price ranges, a $300 dollar build will run something like Garry's Mod just fine, but will struggle to run newer games. A $700+ build will run most thing great with a bit of "future proofing," i.e. able to stay relevant longer.
One thing about PC gaming is that you will find yourself in an upgrade loop. Its not like buying a new console because thats what new games are being released on. Its a conscious choice when you feel you want or need it. For example, let's say you already own a PC but don't like the 30fps you are getting on newer games, after becoming accustom to 60fps over the past few years. You can choose to ride it out, or buy a new card. These cycles are typically faster then say upgrading to a new console every seven years, but again you upgrade as you see fit. In a sense, you choose when the new gaming machine gets released for you, which I find very appealing. To give you an idea of what that may be like:
I started out by buying a prebuilt HP with a decent foundation, i.e. CPU, amount of RAM, etc.
I then got a new power supply and a new graphics card.
When the CPU started showing its age, two years later, I decided it was best to build a PC from scratch.
I built a new PC with a powerhouse of a CPU, GPU, enough RAM, an overpowered power supply, planning for the future.
After about two years, I was no longer able to max out games without question (this is my preference and there is nothing wrong with not being able to max games out), so I decided to buy another GPU, running the two in SLI.
Its now been two years since my last upgrade, but I am planning to buy two of the next flagship GPU's come next year.
I dictate my access to new technologies rather then waiting for MS or Sony to design something new. Its a choice every PC gamer makes for themselves, and is something you will become familiar with as you froth out the mouth over the new GPU that just came out. Its a quite liberating feeling having the control after switch from consoles myself about 6 years ago. I never looked back, and don't plan on it.