There's different armors to be had, and also a few extra items that can be equipped. You have an inventory so you can carry more than 2 weapon on you at any time.
Stalker is almost like an rpg, minus skills and stats to improve while you go. Frankly, I'm probably not talking it up enough, it really is a fantastic game. That said, it's atmosphere is awesome, and I enjoyed its story. So, pretty straight forward as far as the mechanics go.
Fairly fast paced fight here, run a bit, fight some more, etc ('course, i guess everything's kinda like that so.). Nothing 'upgradable' that I can remember (maybe ammo carrying 's been a while). Metro is akin to your 'standard' two weapons carried (three maybe with a sidearm? can't remember), no armor or special items to ever be equipped.