Listen I get it you don't have much money, air pods or a PC that can open chrome without catching fire
Well now I have made a guide just for you, so that you can game or maybe edit videos on a budget that doesn't void your wallet.
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Step 1: Price hunt
part a) Firstly you need to aim for a prebuilt within a shop or online under £150 and look for something containing a:
4 Core CPU (preferably over 3GHz) or higher E.g. an i5-2400
8 Gigabytes of RAM or higher
Extras (not needed but desired):
Motherboard with extra RAM slots for the ability to upgrade later
DDR3 RAM slots so its cheaper to upgrade later
I found this - https://www.amazon.co.uk/HP-Professional-Antivirus-Certified-Refurbished/dp/B071H9RP12/ref=sr_1_13?s=computers&ie=UTF8&qid=1549927261&sr=1-13&keywords=quad+core+pc&refinements=p_n_feature_browse-bin%3A1481781031
part b) Secondly aim for a graphics card that will fit in your case dimensions for less than £160 without picking up a fake, such as a:
GTX 1050 Ti 4GB (Usually between £80 - £140)
GTX 1060 3/6GB (usually between £100 - £180)
GTX 970 4GB (Usually between £90 - £190)
Info : The 1050 Ti performs the worst (by a very large margin) but draws the least power, the 970 performs better than the 1060 3GB but the 970 and 1060 3GB perform worse than the 1060 6GB, and the 970 draws the most power (1050 ti - 75W / 970 - 145W / 1060 - 125W)
Part c) Work out the total cost (should be around £300)
Step 2: Power
part a) work out the amount of power your PC will draw at its max, and compare it to the power supply within the pre-built
Part b) Order a new power supply that is above the max your PC will draw e.g 500W (Only if it already surpasses the current power supply)
Info:
if you're new PSU doesn't have a 6pin/8pin connector (e.g if its modular but didn't come with one for some reason) AND you have a 1060/970, order a cable to power your Graphics Card
Step 3: Assemble
Replace you're power supply for the new one, here is a video demonstrating how to do so https://www.youtube.com/watch?v=iFVhgiwJE6U
Connect your GPU to the top PCI-E slot of the motherboard, here is a video demonstrating how to do so https://www.youtube.com/watch?v=vyNe0sSvifE
Boot it up and install drivers from geforce experience or nvdia's website
Now for upgrade-ability, your options are:
Buy a new CPU Cooler to prevent thermal throttling (E.g. a hyper 212 evo for £20)
Buy an extra ram stick if you have spare slots on your motherboard (or replace one with a larger storage)
Buy a £20 SSD and move your Operating system to it for faster boot times
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And you now have a £320-£400 PC for gaming, editing, whatever you desire.