In gaming once you pass about the i5-2500 there are few frames to be had as you go up in CPU speed or power. As to overclocking, not a fan as I see such machines later and we have to set them back to stock to stop odd BSODs.

That aside, check overclock forums about ideas on what board and cooling. Many go to water cooling or cryo to get high clock rates. But gaming won't benefit much.

If this was my PC I'd dump the HDD and put that money into the next sizes up on the SSD.
As to cases, I don't spend much there. Nod to what https://www.reddit.com/r/PCMasterRace/wiki/builds writes about that.