Did you turn off the in-game MSAA? Only it says that it doesn't work if this option is turned on.
Yes I did, I don't think the instructions are very clear. Sorry to ask this of you but could you please help me. Tell what you installed and where you extracted files? Do you need to install the patch with the gog version as well?
I don't have the game on GOG (or in fact on PC, I have it on the original Xbox and played it quite some years ago now). I'm watching the forums to see if anyone can confirm that the game-breaking bug towards the end of the game (in the tunnels where you have a time limit to escape) has been fixed, and if not to see if GOG eventually do get it fixed. I'm very interested in getting the game here, but only if it will be able to be played through fully without using third party programs or cheats.
From what I've read, though, I gather that the GOG version has a number of fixes applied so I don't think you need any patch. I'd say install the GOG version, then extract SweetFX to the game folder and set your settings in the ini file, then start the game and ensure that MSAA is turned off in the game launcher (if you still want AA then I've read that you can force it on via your graphics drivers instead of the game settings).