I'm accustomed to designing for mobile hardware, where I just feel like expectations are lower than on PC.

However, I'm going to be using the Unreal 4 Engine for a PC game it looks like, so I might as well try to make it look good.

The problem is, like I said, I'm accustomed to designing for mobile. I will have more power available now. But I need to learn to utilize that extra power well - I mean, you can't just put 1 million polygons in a rock.

So besides my general "Got any advice?" question, I'll also ask a more specific question. It's almost a proven fact that models alone won't make a game graphical. You need lighting, etc. But my question is, not counting normal-mapping and completely forgetting about it for a second...

Would I be okay making human characters 20k polygons for a modern PC game, and focusing on lighting and stuff to make it graphical, or do I need something more robust like 40k per human character?

I'm aware the actual mesh and its quality matters as well...

Any advice is appreciated.

I'd say you're pretty decent in the 20k polygon mark, but I guess it also depends on how many of them you want to render simultaneously. Afaik, and just for a reference, Hitman Absolution's main character (2012) and Tomb Raider's Lara (2013) are in the 20k-35k range.