iMac Combo Headphone/Optical Out Port Cable

Some standard toslink cables come with the adapter to fit the headphone output. Mine did. It's just a small piece of plastic that looks like a headphone jack on one end and accepts the toslink on the other.

I've used this adaptor with excellent results:

Toslink To Optical Mini Adapter

Or you can use something like the Wireworld Supernova 6 which is toslink on one end and mini toslink on the other.

there are quite a few cables out there that you can use. depending on quality, you can get them from apple, monster, and ixos. the mini adapter can be purchased from fry's, radio shack, echo audio, and others. i purchased a few from fry's for $2.98.

mezmo - stay with the older PC technology.

As others have said, neither the analog nor the optical output on the G5 uses a proprietary cable. You can use either a 3.5mm stereo plug for analog output or a mini-Toslink to Toslink for digital.

If your question is which one is preferable, it's likely that the digital to analog converter in your receiver is better than the one in your iMac so that's likely your best option. The cables are inexpensive, though, and it would be easy to compare. A good source for well-made cables at great prices is monoprice.com.