MCP-T has an DSP (Digital Signal Processor) called the nForce APU (Audio Processing Unit). Normally, sound codecs rely on the CPU to process sounds, but high end sound cards contain DSP's just like the nForce APU. These increase system performance by reducing CPU overhead, while adding features.
So in this case using the AC97 codec shouldn't hurt system performance because the nForce APU is doing the same amount of processing either way, with the same smaller load on the CPU.
As for sound quality, in this case the Codec is simply a device to split the combined digital channels into seperate analog channels. Such a device also exist inside "digital speakers". So both the board and digital speakers convert sound to analog using similar devices. Therefore, cheap digital speakers with cheap electronics would do a worse job than your onboard codec, while high end digital speakers may do a better job using a higher quality converter.
So you don't really loose quality using the analog output, as compared to typical (read inexpensive) digital speakers, assuming the drivers themselve are the same quality.
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