In the Crysis forum they were talking abt SLIs of high end for smooth gameplay. But also they were saying that a 1 GB RAM and 6600GT with work without the following things:
Soft Shadows.
Volumetric Clouds.
Direct X 10 Features
The HDR Rendering will be smooth but if you put Real Time Ambient Light than the HDR will be slow.
But still what you get with 6600Gt and 1 GB RAM is
Dynamic Day and Night Sun Cycle.
Motion Blur.
Depth of Field
Fully Interactive Environment
Shader Model 3 full support and lots of other features.
The Motion Blur and ambient features are also included in X-Isle mod of Far Cry. You can check out the Demo in the single player map pack1.1:-
http://farcry.filefront.com/file/Sin...Map_Pack;53449
Warning: Size 734.96MB