movie 用 AVI 格式好食 ram ????

AVI is essentially an uncompressed video format which takes up a LOT of space. One minute of video equals ~33MB of data.
I am not an expert on videography etc., but I'll try to answer your questions:
1. Record less, or record the video at lower resolution;
2. I am not sure about this one. Most DCs support AVI recording only, some newer ones (such as Casio) support MPEG4 H264. I am not informed of any other video formats being endorsed in DC's video recording feature;
3. That depends on format and quality settings. At equivalent duration and quality settings etc., the MPEG H264 should take up less space due to the use of higher compression algorithm on the video signals;
4. Don't know how to answer this, there is no particular standard on this;
5. Any DC with video recording function will NOT give you good results. Bear in mind that a DC is designed and optimised for still image recording; video recording function is just a bonus feature which is better regarded as a gimmick.
6. Again, I am not sure about this. But I think those Sanyo models out there might give you slighter better results, due to the fact that those models are designed, configured and (somewhat) optimised for video recording, although their internals are still based on DCs.
If you really need to record video, then go get a real camcorder.
Hope that helps.