want to buy a new pc ,play mostly games

You have to PAY TO PLAY on PC side, and that means AT LEAST $700 for a decent PC to play 'like I see on Youtube' any and all titles that are out.
I am talking 1080P, HIGH+ Graphics, giving 50FPS, usually you want one centered around a i5, with either a AMD 290X or a Nvidia 960.

IF your going to be stuck below that, you will not be able to play games except on MINIMUM at BEST (if it even plays) which is to say
crappy 1024x768 display LOW ugly graphics settings and be thankful to get 15FPS or better.

The alternative is to switch to a PS4. For that price you can play GTA V 1080P High and get 30FPS, as well as many other titles at 60FPS. Further you get a FREE Bluray (3D!!) player, Free Gamer's Headset, and FREE Wireless Controller which would be another $200 onto the price of any PC you tried to have the same hardware on.

Do also know, if your 'OS' you have came out of a Dell, Gateway, Acer, etc. you can NOT REUSE IT in a new PC. It is LOCKED to the hardware it came with ONLY, and will refuse to install / boot, so that is another $100 on top of your costs for a Win 8 System builder edition, or a 'off the shelf at Walmart' copy at $179 .