ibook_steve is correct, although it is worth noting that 5.1 audio over optical only works for movies. Games on the Mac (and most PCs) will not get 5.1 audio that way, although it works on the Xbox. The reason is fairly convoluted, so I won't bother with it unless you're really interested.
Upconverting stereo sound to 5.1 is done using a special algorithm, of which there are two: Dolby Pro Logic II and Neo:6. The former is far more common. It works very well for music and is a nice boost for movies as well, but it's not a patch on real 5.1 audio.
The Logitech set you linked to supports both regular 5.1 decoding from optical and Dolby Pro Logic II, so it seems to suit your needs. You will run out of inputs, however - only one optical in - so you will either need to pull plugs and switch around or get a manual switch.