firstly... read the doc:
http://ffmpeg.org/ffmpeg-doc.html#SEC7

secondly the letter in between of width and height is a x.
not any strange x-like sign. ^^

thirdly i would use the whole screen resolution as far as possible in respect of the aspect ratio of the source.
in your case 1280x720 -> 800x450.

at last i would recommend using the mp4-container instead of avi-container and ask google to search for settings of the iphone videos because it is using the same encoding - just different resolution.

something like this:
attention:
ubuntu's ffmpeg may not contain the aac-codec due to license issues. you may have to compile it yourself with aac or use mp3 (as ac3 isn't well-supported in n900)