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In most cases the line output is enough for most headphones. If you want to use some hi-fi big headphones you're better of with an external headphone amp which is more appropriate for hi-fi headphones than an internal amp.

The G5 does not have an internal amplifier!
W

Pardon me - but where did you find this little tidbit of information?

Do internal amplifiers cost that much ? I mean even an iMac has one. Why ?

Probably a few reasons.  Heat and space being the biggest two.  Price is probably last on the list.  They shouldn't cost that much to include.

From Apple's G5 Tech doc

The sound circuitry is connected to the K2 IC by a standard I2S (inter-IC sound) bus. The K2 IC provides DB-DMA (descriptor-based direct memory access) support for the I2S port. The Power Mac G5 circuitry consists of two main components: an analog audio CODEC IC and a digital audio CODEC IC. The analog audio CODEC IC includes an A/D converter, a D/A converter, and digital audio processing functions. The digital audio processing functions include output equalization, dynamic range compression, and volume control. The equalization and dynamic range control functions are set to fixed values to equalize the internal speaker. Those functions are bypassed for signals sent to the audio output jacks.

The codec functions include one stereo input pair, two stereo output pairs (line out and headphone out), and one mono output that drives the internal speaker. Stereo signals from the audio input jack are routed to an analog line input buffer that drives the internal A/D converter. The output of the A/D converter is routed to the K2 IC over the I2S bus.Digital audio data from the K2 IC is routed to the D/A converter on the analog audio CODEC over the I2S bus. The analog audio output from the D/A is routed to separate amplifiers that drive the line output, headphone output, and internal speaker.The digital audio CODEC IC includes an S/PDIF (Sony/Phillips Digital Interface) digital audio receiver, a sample rate converter (SRC), a phase-locked loop (PLL), and an S/PDIF digital audio transmitter.The S/PDIF input can operate in synchronous and asynchronous modes. In synchronous mode, the PLL in the digital audio CODEC locks on to the incoming S/PDIF data rate and drives the audio system clocks in the Power Mac G5, allowing bit-accurate data processing of the S/PDIF input data. In asynchronous mode, the incoming S/PDIF data passes through the SRC, where it is converted to the output sample rate of the Power Mac G5, allowing the Power Mac G5 to play audio at a fixed output sample rate while accepting S/PDIF input data over a wide frequency range. The S/PDIF input data is routed to the K2 IC over the I2S bus.Digital audio data from the K2 chip is routed to the S/PDIF output transmitter in the digital audio CODEC over the I2S bus. The Power Mac G5 sends and receives S/PDIF audio data over TOSLink optical connectors.Analog audio I/O is not independent of digital audio I/O on the Power Mac G5. Audio input can come from either the line input or the S/PDIF input, but not both at the same time. Audio output can be played simultaneously on the line output, headphones output, and S/PDIF output. However, these audio streams are not independent.For more detail on the audio, see

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