Need help finding a sound card

I recently purchased the Xonar DG because when I looked up the specs of my motherboard I saw that it had a PCI slot. I knew I should have actually looked inside my case just to be sure, but I didn't, and it turns out my motherboard only has PCIe slots.

The reason I picked the Xonar DG was for its headphone amp since I recently purchased a pair of Ultrasone HFI-580's and figured that it would be a good idea to get a sound card with at least some kind of amp in order to get the best playback in these new headphones. I also picked it because I thought I was going to have to splurge out more money on a new speaker system as well, however that's no longer a problem. Now I have a bit more money to spend on a sound card but I'm not sure which one I should get. I'm looking for something that will hopefully have a headphone amp on it in order to drive my new phones better, and will produce significantly better sound than my onboard audio. I'll be using it for both music and gaming, but mostly music (probably 80% music/20% gaming).

I'm not really sure how much I should be willing to spend for something like this since the DG was only around $25. I don't think I'm ready to spend over $100 for anything though.

I found multiple designs of the U3 here, not sure which one you're referring to. Also what's the performance difference between a USB sound card and a PCIe/PCI/etc. sound card?

I have an EVGA P55 Classified 200 motherboard. They don't make them anymore and I got a good deal on it when I first build this computer.

I looked more into the U3 and I think I may be looking for something a bit better than that. It doesn't matter to me if it's USB or PCIe, as long as it's not PCI since I won't be able to use it.