I've been trying variations of this command for converting my flv's to mp4's
ffmpeg -i "original_video_file" -vcodec libx264 -s 320x240 -r 20 -g 250 -keyint_min 25 -coder ac -me full -me_range 16 -subq 5 -sc_threshold 40 -acodec libfaac -ab 96000 -ar 22500 -cmp +chroma -partitions +parti4x4+partp8x8+partb8x8 -i_qfactor 0.71 -b_strategy 1 -crf 30 -y "converted_video_file.mp4"
"Adjust the size (-s), framerate(-r), and keyframeinterval(-g) to suit."