When I bought penumbra I had the 7600GT. I recently upgraded to an 8600GT. So far the game runs rather well. The only glitch that I noticed was in the room where the "thirsty beast" is and the floor was a bit too shiny.
Well I've played the Overture demo (Short as it is) and am running a Geforce 8600GT. No problems here, just some barely noticeable slowdown in some parts when I had motion blur enabled. Not that I'm taking motion blur off. It looks too good! xD