I dont really know your cpu but I would say that is one of your problems. Stalker has a very strong and in-depth AI system and that takes a good cpu to run. Is that a dual core? Im no expert but thats one thing I see in your specs.
Oh and your RAM is only DDR which will slow you down a bit in these big worlds.
Oh and your RAM is only DDR which will slow you down a bit in these big worlds.