A dedicated DAC should sound better than most integrated DACs, especially one inside of a laptop due to components used, the type of power it gets, and the interference you get from neighboring electronics.
I think it all comes down to how you listen. I don't know that my wife or kids would be able to tell the difference between good and better sound, not so much because they can't, but because they don't care to listen for the differences. So if all of your listening is casual, you may never notice. But if you try to pay attention to more than the sound as a whole, if you attempt to make out the sounds of different instruments or vocals, and the qualities of their individual tones, and their placement within the stereo field, etc., etc., then a good DAC can reward you with better imaging, less veiled sound, clearer less muddy bass response, and other benefits.
But will you notice? That all depends on how you listen.
As far as having old equipment goes, I've been surprised at how good some of the older stuff sounds. Sure some of the amps have come a long way recently, but I've spoken to folks that have had a very hard time replacing their old (decent) amps with newer stuff for a reasonable amount of cash.
For myself, I had an old pair of (quite good) bookshelf speakers, now about 25 years old, but that still sound better than most speakers running upwards of about $800 a pair.
As far as to needing a Dolby decoder goes, I'll hazard a guess only since no one else has answered. Dolby decoding these days is primarily for multiple channels, i.e. beyond stereo. If you're not using more than 2 speakers, I don't think you will see any benefit.
Good luck with whatever you decide.
Dan
