In a nutshell its very very ambitious.
Most people would say it's unrealistic, and I think it's high time that someone over there figures this out as well, and lowers their aim a bit. Once more Doom 3 / Source engine games hit the market, Stalker is going to look like ass.
Yes, I know, looks aren't everything, but it will still be a major deterrent, especially since you're supposed to be wandering around this massive area most of the game. If you have to wander around, there better be something to look at.