well i am using an ixus 55 for video recording (poor man no momney buy dv cam yet)

anyway there is this 640 or 320 settings (think its the pixel size frame? and there is 30fps and 15fps

i tried mixing these

30fps * 640 and 15fps*640 (notice not much diff in quality)
so theoretically 30fps supposed to be better than 15fps....but how come i dont notice much.....anyway wanna ask some suggestion, of course bigger is better, but memory space is limited.......so what is the "average" good quality to the human eye which will produce good + usable material?