I was actually just looking into this same basic issue over the weekend. I'm not a huge audiophile, but I appreciate good quality sound and have spent probably the better part of $1000 on a receiver and set of speakers.
Everything I looked at said that dedicated sound cards do provide a noticeably better sound compared to the average integrated sound. The general consensus is that they have tighter control on fidelity, including bass. So considering you seem to have spent at least a few hundred dollars on your setup, I would say it's definitely worth spending $50-$100 more on a decent sound card. If you were just using a set of basic stereo, or even 2.1, desktop speakers, integrated audio would probably be fine.
If you want to send me a PM in a couple of days, I might be able to give you a better idea. I have a Xonar DSX on order with Amazon, and could give some subjective opinions on the differences. I also think Asus and Creative are about the only companies making actual sound cards anymore. Integrated audio is generally "good enough" for most people, so there's not as much demand for actual sound cards anymore.