Hmm I think that it can be worthwhile if you really buy great equipment. In the home theatre department, I've heard pretty standard quality sound varying greatly in price with little difference in actual sound quality. I have a very basic receiver at home with 5.1 sound, and it's a massive step above tv audio, and a good step over 2.1 but it's not tremendous. Then, I heard my uncle's sound system - top end harmon kardon receiver and 6 foot front towers and 5 foot rear towers and a big ass sub, the box was like a coffee table. And yes, you could really, really hear the quality. Of course, that was probably a $15k system. That's way too much for most people to spend, obviously.
Still, comparing my pretty crappy 5.1 system ($300, 300W, plus I bought new center, left, right speakers for about $60) to my dad's system ($1500) it's pretty significant. Crisper, cleaner, bassy-er. With mine I hear sounds "coming from behind me" with my dad's I hear sounds coming from 12.8 feet, 31 degrees to my back left lol. The point is, there is a big difference in sound but you need to be able to spend money on it.
I'd like to have the extra disposable income to spend $1000 on a really nice sound card, receiver and 5.1 speaker set up to use for PC and PS3 but that's just not feasable for me. So, I might go buy the $100 Logitech X-540 5.1 system which will be a really big step over my 2.1 system but to pair a sound card with that will probably be a waste given the onboard high def RealTek audio.