I currently use a Hercules Guillemont Game Surround Fortimissmo III. It is the new 7.1 channel version, with optical out. I believe it does work under fine under Linux (I installed Mandrake a while ago to test some other stuff, ultimately uinstalled for reasons not related to the sound card, but if I remember correctly it had no problem using the basic line-out).
THe card is very nice with my HD570's. Sometimes a little quieter than I would like (only sometimes), but I can still use my HD's as mini speakers if I want

So, in short, it drives them fine at all but the most extreme of volume levels. They sound fine with CD's and compressed music.
The one thing that I really love about them under Windows is the software. A) it works flawlessly and installs no problem. But the biggest feature is that you can easily switch from 2.1 speaker, to 7.1, to 5.1, to headphones. Much nicer than having to unplug my speakers when I want to use my headphones. I think most newer cards have things like this, either headphone-out's on a breakout box or just on the card.
I do not know how one would turn off line-out and just use headphone-out under Linux though.
But, I highly recommend this card. I paid $44 shipped from newegg when they first came out (like a month ago), this was for the OEM version. I believe they are now at like $45 + S&H for the OEM.
To compare: I had a Sound Blaster Live Value 4.1. It worked fine with the headphones (I think they drove them just a little better, but I have no real data to back this up), with the exception that I had to reach around my computer, unplug my speakers, and then plug in my headphones in order to listen with my headphones.