This should do it: 

ffmpeg -y -i input_file.avi -an -v 1 -threads auto -vcodec h264 -b 500k -bt 175k -refs 1 -loop 1 -deblockalpha 0 -deblockbeta 0 -parti4x4 1 -partp8x8 1 -me full -subq 1 -me_range 21 -chroma 1 -slice 2 -bf 0 -level 30 -g 300 -keyint_min 30 -sc_threshold 40 -rc_eq ‘blurCplx^(1-qComp)’ -qcomp 0.7 -qmax 51 -qdiff 4 -i_qfactor 0.71428572 -maxrate 768k -bufsize 2M -cmp 1 -s 320×240 -f mp4 -pass 1 /dev/null

ffmpeg -y -i input_file.avi -v 1 -threads auto -vcodec h264 -b 500k -bt 175k -refs 1 -loop 1 -deblockalpha 0 -deblockbeta 0 -parti4x4 1 -partp8x8 1 -me full -subq 6 -me_range 21 -chroma 1 -slice 2 -bf 0 -level 30 -g 300 -keyint_min 30 -sc_threshold 40 -rc_eq 'blurCplx^(1-qComp)' -qcomp 0.7 -qmax 51 -qdiff 4 -i_qfactor 0.71428572 -maxrate 768k -bufsize 2M -cmp 1 -s 320x240 -acodec aac -ab 96 -ar 48000 -ac 2 -f mp4 -pass 2 output_file.mp4

 ffmpeg -y -i input_file.avi -v 1 -threads auto -vcodec h264 -b 500k -bt 175k -refs 1 -loop 1 -deblockalpha 0 -deblockbeta 0 -parti4x4 1 -partp8x8 1 -me full -subq 6 -me_range 21 -chroma 1 -slice 2 -bf 0 -level 30 -g 300 -keyint_min 30 -sc_threshold 40 -rc_eq 'blurCplx^(1-qComp)' -qcomp 0.7 -qmax 51 -qdiff 4 -i_qfactor 0.71428572 -maxrate 768k -bufsize 2M -cmp 1 -s 320x240 -acodec libfaac -ab 96 -ar 48000 -ac 2 -f mp4 -pass 2 output_file.mp4 

Edit: second command was wrong at least on gentoo, the -acodec was unknown. Replace acc with libfaac instead, make sure you have acc in the USE flag for ffmpegSettings do work!

 Run the first command and then the second for a multipass encoding

 Please leave a comment if it also works for you! (no registration or real email needed)