Run the following command (or create as a script) in the folder containing the images you wish to batch process.
for file in *; do convert $file -resize 800x480 output/`basename $file`; done
This might not make full use of multi-core / multi-processor systems as it processes the files sequentially but I've found that passing *.jpg to convert (part of ImageMagick) can use up all of your RAM and swap space.