Things to get Clear Sky running as smoothly as possible
Update your drivers. If you are using an ATI get the Beta Drivers (V8.54-RC2) for the upcoming Oct release of Catalyst. They are essential.
1) Do not use the "-noprefectch " commandline switch. It worked for SOC but will increase stuttering and freezes in CS.
2) Since the game will not utilize both cores of a dual core processor, you need to base ALL your settings on the video card's capabilities. Your processor will not help you very much. If you are using a card that is ranked lower than a Radeon 3850 then you should probably stick to static lighting unless you are willing to turn almost all graphical setting to 25% or lower.
3) The screen resolution (eg. 1280 x 1024) has a large impact. What will not run smoothly at 1280 x 1024, will run nearly perfectly at 1024 x 768. Start here and tweak the rest of the settings to your tastes.
4) Given #2 (above) you only need to make sure that your background programs (firewall & AV software) are not in active scanning mode. You should have plenty of CPU power even with older dual cores (<2.8 GB) to handle running them with the game. Running a file server in the background is a different matter. ;)
5) The graphical level is the largest determinant of the performance hit. The DX9 and DX10 versions of "Enhanced Full Dynamic Lighting" are identical except that DX10 comes with God Rays and Rainy Surfaces. The God Rays are horribly implemented, since they continue to effect performance even when they are not visible. These need to be tuned down between 6 am and 8 am in the game. In SOC it was easy to make most changes to your graphical settings, but for some inexplicable reason they decided to make this as painful as possible. You need to restart the game for most changes to take place now, and it does not always warn you about it. I would suggest grabbing a time acceleration mod, to quickly skip from 6 am to 8 am and bypass this colossal bug if you are going for the full-out DX10 EFDL.
If you are looking for something a little less demanding then start with DX9 EFDL and reduce from there. I would also suggest testing the graphical level on a resolution of 1024 x 768 to reduce the stutters and slowdowns, while testing the various settings on your system.
6) Try decreasing/disabling AF, Shadows, , AA, and Dynamic Music for a fairly large performance increase. I would reduce them in this order, until you get it running the way you want. Then try and trade-off an increase in one with a similar decrease in at least one of the others.
7) If you still need more, then reduce vegetation, view distance, and texture detail in that order.
8) Do not use mods that have better textures unless they state that they are not larger images. If you try and use higher resolution textures, it will impact performance.
9) Last ditch effort - use the "-nodistort" switch to remove the particle warping effects, but beware the dangerous areas. ;) This may stop those nasty CTDs in areas with a lot of distortion effects, if you are getting them.
Generally, if you are using Vista with 2 GB or less ram and a less than awesome video card, do not try dynamic, do not pass the DX9, go directly to DX8. Vista eats up over 1 GB of that RAM itself (and much more if you have not tweaked it). That means that your system is easily bottlenecked and it will not run well for you. If you do have an awesome video card with that kind of system then you should get some more RAM. It would make a huge difference for you to buy two more sticks of the particular flavor of RAM you already have (assuming that your motherboard has a four slot configuration).
Advanced (from Tweak Guide)
"r3_gbuffer_opt [on,off] - This setting appears to control a form of potential rendering optimization for DX10 mode. If set to on it may improve performance, however it is likely it has been disabled by default because it can cause glitches, crashes or due to other bugs. In my testing it didn't appear to cause any problems, but it also didn't seem to improve performance or image quality either. You can experiment to see if it helps you."
If you are getting errors then go into your user.ltx (C:\Users\Public\Documents\STALKER-STCS in vista) and change it to off. While TweakGuide says it comes disabled, it is enabled on my install with patch 1.05.
The Opposite of Faster
If you want the game to run slower but look better then use one of these switches to increase the size of the texture used for shadows.
-smap#### where you replace #### with 1536, 2048, 2560, 3072, or 4096.
Using this switch will save screenshots in tga format as well as jpg, with the tga files being high quality. Not so useful if you are using FRAPS, but if you just want some no-hassle good pics, it is handy. -ss_tga
Minor correction - god rays can be activated on Enhanced Dx9.
The only features entirely dependant on Dx10 are Wet Surfaces and Volumetric Fog.
Also, is it just me, or does Anti-aliasing not work on enhanced?
It does but only in DX10 mode after patch 1.05.
I just experienced a CTD and the xrEngine.exe*32 process was still running! Of course this prevents you from re-launching the game. There was no other window so Alt-Tab and Switch Between Windows did not work. If you experience a CTD and cannot relaunch the game then open your task manager (Ctrl+Alt+Del) and shut xrEngine.exe*32 down.