find -maxdepth 1 -name "*.jpg" -printf "%f\n" |
sed 's/[^.]\+/&\.jpg -resize 320x240 &/' |
tr '\n' '\0' | xargs -r -0 -n1 -P4 convert

Re: http://lbrandy.com/blog/2009/10/this-must-already-exist/