First, go into the exe's properties and type -noprefetch in the Target line, outside the quotation marks. Prefetching was put in for when it was going to be open-world instead of zones, and now it's the cause of all the lagging, choppiness, and assorted graphical frustration that the game brings. Before I knew about this, I could run the game on my reasonably powerful computer only if I turned off dynamic lighting, and it was still insanely choppy to the point of unplayability. Once I did it, I can play it without dynamic lighting with everything else maxed, or with dynamic lighting and moderately good texture quality'n'such. If that doesn't work, there's also a registry tweak, but I don't remember it. You can probably google it.
Mods: Most of them are not compatible with each other, so you have to pick one, although last time I checked Alive and Redux could be put together with a separate download to fix the things overwritten.
Alive-An A-Life mod that unchains the animals and NPCs, allowing them to move according to their own whims. Bloodsuckers in Cordon, Nimble moves to the Dark Valley before you can get his flash drive, good for people who like difficulty.
Redux-Realism mod, no A-Life changes. Makes things much harder, NPCs bleed, on the harder difficulties you can literally be killed in one hit, armor degrades rapidly, tracers, some ammo is corrosive, guns are found in terrible condition, bleeding is a serious problem and bandages/food no longer restore health. Food goes from annoyance to gun in your face, you get hungry and hour after eating. Very difficult, but originally the Mod I Couldn't Live Without.
Oblivion Lost-Best mod ever. Does the same thing as Alive and Redux, simply not to the extent that they did it, it's still hard though. It attempted to make the game more like what they said it would be, and to this end adds cars, blowouts, all of the old monsters, new guns, new artifacts, more randomness, all kinds of stuff. So many epic moments in this, even from day 1. There was the time I saw a body in the woods by the road and stopped my car. I jumped out of the car, and pulled out my binoculars to see who it was. Suddenly I hear The Panting(The Panting of a Bloodsucker, one of the most terrible noises you can hear in the game). All I had was a pistol. I jumped back in, floored it, and drove away so fast I flipped my car just as two bloodsuckers jumped on it. Thankfully there was a guy with an AK-74SU nearby, who distracted them. Blowouts are also all kinds of awesome. They affect NPCs, which is why originally they didn't put them in, they kept killing everyone. Now NPCs run to cover, which leads to some intense shootouts as rival groups of STALKERS rush for the same building. After a blowout it's storming like wtf, and zombies come out of the woodworks. Some of the additions are dumb though, like artifact enhancement. You throw certain artifacts into certain anomalies, and form new artifacts. I don't think it works. I certainly know I never got my Droplet back, and that's like the best artifact ever.
Also, read Roadside Picnic, see how much cooler that zone would've been compared to this one.