The 8320's are hot chips and it's down to the voltages etc on them, it makes me laugh these days as chips are getting hotter and hotter when they should be getting cooler.
Thermal paste should be Artic MX4 or Noctua NTH-1, as they are the best you can get without going for stupidly expensive better by 2c stuff, like the Ice Diamond stuff.
Temp monitoring software is always a issue with AMD cpus, as most pick the temp up for the first core and replicate it across all the cores, but the best one iirc is Hardware Monitor.
Coolers will be difficult to fit, but the best ones out there for your chip if you want to stick with air are:
1:
Phanteks
2:
Noctua NHD-14
3:
Noctua NHD-15
4:
Be Quiet Dark Rock
If you really want to drop the temps though then you would be looking at either a Corsair H80i or H100i.
Any of those coolers might be abit tricky to fit given your situation but you could always, ask someone who knows what they are doing to do it for you or take it to a local computer shop, as ask them how much they would want to fit it.
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