Best performance, best graphics. Looking for tips.

I was looking forward to create some sort of API where I can render really good last-gen graphics within Unity3d, but with the lowest cost of PC performance. I'm talking about perhaps 1-2 y/o computers with somehow a minimum of 256 MB of VRAM for example, which cannot render Crysis 3 for example.

Yes, I know I'm not Carmack.

Yes, maybe I got to do the research myself, but I'm still asking for guidance on this just in case I'm missing something. I plan much not only on beautiful graphics with lowest requirements, but also on FPS gameplay, tech and accesibility.

Anything?