At 20 fps you're going to see some problems... mainly the human eye tends to react best to 22 fps and above. 24 fps is standard for motion pictures and 30 fps for television. Once you exceed 22 fps, every increase in image density will begin to affect how smoothly the animation appears to run. My advice is to run at 30 fps unless you're working on experimental techniques that are intended to cause psycho-active reactions, such subliminal splicing, color cloaking, etc. That should solve part of the issue, however... In terms of compression for an .avi, unless you have an enormous amount of detail that will be lost by your codec, there's no reason to keep pushing the compression down unless you just love huge file sizes. I have 16 sequential renders of an animation I'm working on, and without a doubt the difference between the same sequence rendered at 30% compression and at 70% compression is close to negligible. If you're pressing to DVD for television viewing, this difference will diminish even more due to the dual-scanline rendering process that television uses, which actually uses a two pass render and ghosts part of the image for a short time. The result is the very smooth motion of television.