I agree, roughly. At the audiophile level there is some improvement to be expected with S/PDIF in particular(less cable noise). But the situation today is nothing like with older on-board sound where they used the cheapest generic AC97 chips and the output sounded like a tin can.
If you're recording or need low latency playback, there's still tons of room for improvement by getting a sound card. For good recordings, you need a preamp stage, you need <50ms latency, and you (often) need more than one track. And the entry price for these features is still relatively high, between $100 and $250.
If you're recording or need low latency playback, there's still tons of room for improvement by getting a sound card. For good recordings, you need a preamp stage, you need <50ms latency, and you (often) need more than one track. And the entry price for these features is still relatively high, between $100 and $250.