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.
Creston