I do audio production, so I look at those products and shrug my shoulders, because for a similar amount of money, you could get a full external audio interface with a large number of XLR recording inputs and outputs. But I suppose if you're only ever playing back music, never recording it, I could see an interface like those being superior by virtue of their connection and playback features (and the built-in power amps; most audio production gear assumes you're using self-powered studio monitors).

If you're only ever playing back compressed audio, though (MP3, AIFF, DVD, Bluray), I wouldn't expect the DAC portions to sound noticeably improved, and even if you're playing back uncompressed original source audio, I'd think the change in DAC would be super-subtle. If your current computer sound system doesn't have any current problems with non-DAC related things like hum or ground loops, then I'd think you'd not notice a difference moving to an interface like those. You'd probably get more bang for your buck by putting the money toward upgrading your speaker systems; that's always a place where you can hear immediate improvement.