300 isn't bad, but it's probably a year.
300 Recommended configuration: Celeron g1820(with H81 motherboard) gtx650ti. cpu to buy Taobao loose pieces, graphics card Taobao second-hand, about 320.
If you have a 450 budget, you can get a Pentium g3460 gtx750ti, also an h81 motherboard. Can fight for at least two more years.
Note that the memory selection of ddr3, you can first a single 4g, not enough to add a single budget 120. Power supply 400w enough, budget 150. 20 bucks for cooling is enough. If you only play lol, you can directly play a 120g ssd with a budget of 150. Plus a nice used h81 motherboard for a budget of 150. The whole machine can be pressed to about 1000. Display 1080p of 400, total price 1400 solution.
H81 motherboard supports 1150pin CPU, which is the mainstream product in the market. 1, CPU optional Intel fourth generation Core i5 processor, more suitable. If conditions permit, i7 is better; 2, graphics card, high-end products can be selected. N card, GTX960 or above can be used; A card, R7 270, R9 370 and above; 3, power supply, considering the full load power consumption during the game, and other parts consumption, system allowance, 500W or above rated output power. 4, if it is a game console, the memory is optional 8G x 2. At least 4G x 2. Hard disk naturally can not be less SSD120G.