Stalker Clear Sky help! Thread

hello guys,first of all when i opened up the clear sky installer for the first time,i noticed that in in the compatibility screen,it wrote: Physical memory:1000 (or 1024 something i dont remember exactly) i know that this isnt 8 giga of ram that my pc has...so i installed complete mod it works fine,but my fps are really low,like 30-60(when it hits 30 is really irritating),this is so weird for me cause in SoC with complete mode i was 100-200 fps..and my specs are
cpu:i5 3570K
gpu:hd7970 3GB
Ram:8GB
so thats why im saying this is weird... with that pc im playing everything that comes out in ultra quality with 60 fps steady,please someone enlighten me of why is this happening?