I did a bunch of testing on my machine:
I5 4670K @3.6 and @4.2
AMD 280X
8GB RAM
I tested on the wilderness with mostly just Geralt on screen.
First off, overclocking the CPU from 3.6 to 4.2 did absolutely jackshit for my framerate, not a single fuck was given by the game. Likewise, putting Textures and Detail at Ultra didn't affect a single frame. So go ahead and jack that shit up if you have a 280X
These are the values that affect the game the most (hairworks is always off):
Foliage
HBAO+
Better off using SSAO and Foliage at high at the most if you want something that hovers around 45~50fps. Incidentally, it's foliage (and texture quality, but it's irrelevant performance wise), that makes the biggest difference from a visual perspective. If you use high-ultra texture settings the game still looks pretty nice with everything else on low, and on a 280X you truly get close to a steady 60fps there. If you want a mixture of great visuals and decent performance then put everything on high except textures and detail, which should remain on ultra. You'll get something between 40 and 45fps between depending on scene complexity.
Oh and overclocking the 280X to 1100 mhz gave me 3~5 additional frames, but I'm not sure how safe that is. The temp rose to 70C and the fan speed went to 70%. Chance the machine will hang too, probably. I really should've bought some other card...
But the worst thing about this fucking game is the constant juddering, even when you're running at 60fps. Fucking hell, why. Someone needs to put some fire in CDPR's ass to fix this STAT.
I thought it was an ultra chart, my mistake.