The 'optimal settings' doesn't exist. Every setting has it's own advantages and disadvantages. To get the best result you will have to test a lot of settings on your own.
You can also try to start with a stock ENB setting and develop on your own. Or you take an existing setting which you like and start modifying from there. A lot of that is
system specific and only you can check the performance on your rig. It took me over a year of modding to find the best compromise between visual power and performance.
I always recommend settings from Dpeasant3 or icenhancer is also nice. Choose something that fit your taste and slowly start with editing on the .ini. If you are able to minimize
your game screen while playing you can also make .ini changes while the game is running. That's pretty nice cause you can directly see what happens by changing this or that value.
You have to test if your system can run the ENB typical 'downsampling' otherwise I strongly recommend you to choose a variant with another AA method like FXAA or whatever.
I recommend you to read this carefully: http://gtabay.cba.pl/index_dokumentacja_gta4_en.htm
It's the most comprehensive documentation available I know about regarding the .ini parameters for ENB adjustment in IV. Thanks to Darecki. This post has been edited by NaidRaida on Friday, Jun 1 2012, 18:35 
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