Hi Ryan,
Just connect the 740 line-outs (L+R) to the line-inputs of your computer system.
Most computers have 1 or more stereo mini-jack (3.6mm) inputs. (Perhaps you need a special cable : 2 mono (6,3mm) jacks to 1 mini (stereo) jack)
Almost every soundcard comes with a 'wave-editor' that is able to record.

I prefer Sonic Foundry's Soundforge5.0 but there are many others like Wavelab (Steinberg) or CoolEdit.
Most of the current wave-editors have a 'save as' option and can save in the MP3 format. (Most compact and good quality)

So, after connecting the PSR, adjusting the optimum volume level, start the computer's recording mode and then start the midi-sequencer of the PSR.
When the song has ended stop the 'recording' and save the file as MP3.
MP3 files have sizes between 3 and 5 Mbytes.
I hope this answers your question ?

Roel