Confusing responses...
You have two options for connecting your Macbook to an external receiver/amplifier.
1. Use a 1/8" to 2 RCA cable to connect your Macbook's headphone output to an input on your stereo system.
2. Use a Mini-TOSlink to TOSlink cable to connect the Macbooks's optical output (the same hole as the headphone output) to an Optical digital input on your stereo system.
Optical is a higher quality transmission. BUT, there is a question you need to ask yourself. How nice is your receiver?
If you have an older receiver that was considered very high quality say, 20 years ago, then I would NOT "upgrade" to a modern receiver with a digital input. If you have an old crappy receiver, then a nicer new one with digital inputs might be a nice upgrade if you wanted one.
However, I've got two main receivers. The first setup is an old Kenwood KA7100 receiver from the late 70s, and a Phase Linear 700-II amplifier from around the same time period, as well as some very nice old EAW speakers. Digital audio wasn't even invented when this stuff was made. However, it sounds incredible, and will beat ANY audio equipment you can purchase at Best Buy. You have to spend quite a lot, even today, to beat it on raw audio quality. If you have a nice old system like this, "upgrading" to a digital receiver would actually "downgrade" your total audio experience.
The second is a nice Pioneer modern HT receiver with optical inputs. I use this one for home theater. I do prefer to use the optical input when I'm plugging into it, since it makes sense to use the best input you have access to. But the Pioneer with optical doesn't sound better than the Kenwood/Phase Linear with analog input.
So anyway, thats probably more than you wanted to know, but you basically have two options, depending on the type of stereo you are connecting it to.
Peace,
Sam