cpu or gpu upgrade?, help me please :)

Certainly that's an option worth considering. The only problem would be you have no idea how long second hand components will work. While CPU should not be a problem, but motherboard can be. It would be a real pain to lose a machine to a motherboard failure say 2-3 months after purchase.

That is a dead platform too, although it is a lot better than the 8320. I would point you to the AMD Ryzen lineup for streaming. You don't need 16gb of RAM, 8gb will be enough for CSGO and Fortnite. I would go with this and you will be under 350 and will stream like a champ.

PCPartPicker part list: https://uk.pcpartpicker.com/list/HVcGD8
Price breakdown by merchant: https://uk.pcpartpicker.com/list/HVcGD8/by_merchant/

CPU: AMD - Ryzen 5 1600 3.2GHz 6-Core Processor (£137.94 @ Aria PC)
Motherboard: Gigabyte - GA-AB350M-Gaming 3 Micro ATX AM4 Motherboard (£65.99 @ Aria PC)
Memory: Corsair - Vengeance LPX 8GB (2 x 4GB) DDR4-3200 Memory (£97.50 @ Aria PC)
Total: £301.43
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-05-16 17:27 BST+0100

I was recently reading some of the newegg reviews on 8th gen processors.
I noted that there were several that noted a big difference using a 4 core i3-8100 over their previous FX-8350:
https://www.newegg.com/Product/Product.aspx?Item=9SIAB276SG6918

You can buy a bundle with a i3-8100, 8gb of ddr4 and a z370 motherboard for £269.99
https://www.amazon.co.uk/Coffee-Motherboard-2400MHz-Crucial-Pre-Built/dp/B07BMC1PMW/ref=sr_1_2?s=books&ie=UTF8&qid=1526488603&sr=8-2&keywords=i3-8100+bundle

I don't know if this is what you should do, but it fits your budget.
If your budget is a bit better,
you can buy a 6 core i5-8400 bundle for £359.99
https://www.amazon.co.uk/Components4All-Motherboard-2133MHz-Crucial-Pre-Built/dp/B07CN3TNWB/ref=sr_1_1?ie=UTF8&qid=1526488670&sr=8-1&keywords=i5-8400+bundle

First, though, take the time to do o few tests to verify that a cpu upgrade is what you really need.
Here is my standard text for that.
In particular, test to see how many threads you really need.
For many threads but lower clocks, ryzen is hard to beat.
Some games are graphics limited like fast action shooters.
Others are cpu core speed limited like strategy, sims, and mmo.
Multiplayer tends to like many threads.

You need to find out which.
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To help clarify your CPU/GPU options, run these two tests:

a) Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.

b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
This will simulate what a lack of cpu power will do.
Conversely what a 30% improvement in core speed might do.

You should also experiment with removing one or more cores/threads. You can do this in the windows msconfig boot advanced options option.
You will need to reboot for the change to take effect. Set the number of threads to less than you have.
This will tell you how sensitive your games are to the benefits of many threads.
If you see little difference, your game does not need all the threads you have.



It is possible that both tests are positive, indicating that you have a well balanced system,
and both cpu and gpu need to be upgraded to get better gaming FPS.
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What's your current power supply? Depending on the manufacturer and model of it, you may or may not want to get a new one vs reusing your existing one.

There are supposedly good Corsair models, and bad Corsair models, though which is which, others know WAY better than I do which ones are the good vs the bad ones.