I don't think the "jump" in performance between SoC and CS was too great. They mainly built new things upon the engine, but they kept the old stuff (it still supports DX8 for example). I'm not sure how CoP will be, but I don't think they'll push the requirements too much. Who knows though.
I ended up loving SoC. Granted, I had definetely been hyped up (in the earlier previews) for a totally different game than what was released, but the atmosphere and gunplay was undeniably good for me.
I didn't like Clear Sky much. The atmosphere really did suffer in this game, it no longer had that lonely feeling that SoC could conjure so wonderfully in certain areas. The faction mechanic and the focus on it felt a bit out of place. I could've definetely still enjoyed that though, but unfortunately it was pretty broken as well.
What I did like was the weapon modding system and the anomaly revamp. Also, the gunplay just felt improved somehow, more solid, but without becoming arcadey. There were also minor improvements like how the nights are pitch dark etc.
I'm looking forward to CoP. Hopefully they have learnt some good lessons from both of the previous games. The STALKER series is one of few FPS games that I'm really interested in.
