Your error message suggests that your CPU is overheating and so your slow performance is mainly the result of thermal throttling.
Your motherboard will support up to DDR3-2400, but the DDR3-1866 is the fastest your processor will natively support. A memory upgrade will only increase performance if you also upgrade your processor - and even then not by very much for typical day-to-day usage.
Even the smallest and cheapest SSDs available today are much faster than HDDs and more than enough to run your operating system - Windows 10 (but nothing else) will easily fit onto just 20GB. But I agree with above, it's worth spending a little extra to get 120GB or 240GB or larger capacity because installing the apps/games you use most often onto your SSD helps remove performance bottlenecks.
Your 600W PSU isn't really strong enough to overclock your components, but it should be sufficient for your components under load.
Your Carbide 400R chassis can top-mount a Corsair h105 (thicker 2x120mm rad) or Corsair h110 (2x140mm rad) - they cost a little more than a Corsair h100i but are also better performers.
I think a Cooler Master Nepton 280L or Silverstone Tundra TD02 or NZXT Kraken X61 would be an even better choice - more efficient cooling performance and/or less noise (and sometimes less cost) - than a Corsair h100i/h105/h110. Corsair AIOs have out-of-the-box compatability with the hole spacing and colour/style theme of your Corsair chassis ... but you would have to do your research (and possibly a little modding) to guarantee proper compatibility between a Corsair and a non-Corsair product.
Whatever AIO CPU cooler you choose, it's important to make sure you have good airflow in your small chassis so that the memory and rest of the motherboard don't overheat - I would personally mount the AIO rad as a top-exhaust, while keeping the 2x120mm front-intake fans and 1x120mm rear-exhaust fans where Corsair put them, but if temps seem a little high (especially on the GPU card) then I would upgrade the rear-exhaust fan to a 140mm and move the existing 120mm to bottom-intake.
"All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others." - Douglas Adams
[/Korth]
