If you are serious about buying a gaming PC then go to someone (If you can't build it yourself) who can custom build it for you. Factors when building to look at are; CPU, RAM and Graphics/video Card.
CPU - get the higher end i5 for gaming, not the standard. You DON'T need an i7. That's just overkill and a waste of your money.
RAM - get at least 8GB of GAMING RAM. Like a Hyper Fury Kit.
Graphics card - obviously you can't rely on on-board graphics when gaming. A good card will set you back a fair bit, but it worth it. Asus make some of the best.
You can also have a normal HDD and a SDD. The SDD will have your operating system on it for faster booting/loading. Never have important stuff saved on a SDD as when they die there is NO retrieving data.
As for the cost, it really depends on where you are and what your budget is. Don't cheap out on those three things I mentioned. The rest you can build on later as you go. Like more RAM, Better cpu cooling and such.