Thanks for the reply ajb - that makes a lot of sense. As the arduino only allows a max current draw of 40mA per I/O pin, it looks like I'll need to amplify current (or supply a secondary pwr supply) in most situations.
The mosfet route looks like it'll be far too expensive.
I'm considering http://docs-europe.electrocomponents.com/webdocs/0881/0900766b8088183b.pdf - a 8-channel darlington transistor array. (ULN2803A)
It's rated at 500mA, and as the 480mA is a maximum, I think it should be okay?