Tick "Force lossless RGB capture" from FRAPS's Movies tab and max out all the image quality settings from nVIDIA/ATi Control Panel (I guess you know how to do this?). Also make sure that SA's own image quality settings are maxed out. That's all I can come up with.
I've never played SA with 1920-resolution but according to
this you need to patch the game. Give it a shot.