Dimension E521 w/ Vista Business 32bit , can I get SATA out on this? ( Have 2 hd already)

I would like to know if there would be a way to get a sata out plug in the back of the tower (perhaps a card or something)?

I have a 80 master and 500 gb slave HD's installed now but would like to have a larger external HD to store my music and backups on.

Could this be done, would I have to modify the power supply of anything else? I have empty slots for a card if that is needed.