It depends on what you have to plug those into. Fuller sound isn't really an argument anymore as before that was all in how many voices your chip could handle at once. These days, all onboard solutions can handle 128+ voices in software. All you're paying for in higher end cards is a DAC (if you're running analog out), OPAMPs, and the possibility of having a headphone amp that can support anything over 30ohm. Literally, that's it. All can be had for like 60 bucks in a USB external DAC which also support toslink output. If you have a low end sound setup, it makes any of that pointless. For actual computer sound, I would put my JBLs between low and mid range in quality and even still it would be rather pointless to waste any money on higher end output. I have an X-Fi Extreme Music, and there is literally no difference between it and the codec on my current older board, or the codec on the newer ASRock Extreme 6 X79 I had. Unless you count not needing to install unnecessary nonsense, that's a plus.
If I had studio monitors or the PA setup I had at my old house then it would be worth it. Even a nice high end set of cans like HD650s. My current cans crap out at 17k. Pathetic really.
Another thing to mention is software. Most all sound cards have **** for drivers and real software support for the powerful hardware they come with. I've owned an STX, X-Fi Titanium Pro, Xonar, and a few others along side my Extreme Music. Pretty much all had terrible support unless you found that sweet spot driver.