I HAD the exact same setup as you. I say HAD because my SB external soundcard finally gave up the ghost after about 6 months of use. I had it hooked up to my Toshiba laptop.
First off, volume was poor, I had every volume control set to max, and output was too low for me. For example, if the large fan was on in the room, I had to switch it off to listen to movies with this setup.
Sound quality? well I finally dusted off an old Sony receiver I had, and plugged the 555's into the headphone out, and WOW night and day difference in volume, bass, SQ, everything. Instant grins. Made the 555's sound like the $100 phones they are. Made me think that instead of paying too much for them, that I paid too little! Also made me want to toss the SB ext card into the trash. It ended up dying anyway a few weeks after that anyway, after about 6 months of 3-5 x's a week use.
It was twitchy with Vista also. Had to download drivers from the Creative website for it to work. Also, might be a problem unique to me, but the slightest bump to the USB connection, on any port, would cause output to cease. Only way to get it restored would be to restart the whole system. Very annoying when watching a movie, had to keep very still. AND, would only get recognized about half the time anyway upon connecting to the laptop, whenever I connected it, it would require a restart about half the time.
Overall, I liked the M-audio roadie best of the 3-4 cheapo USB external soundcards. But it only worked about 3 months before it finally cooked too.
I still have the 555's, just trying to get a good source for them. I also have ipods and a Zen Vision M, and the sound from them doesn't begin to compare to the sound from my old Sony receiver's headphone out.
I would say the 555's are HIGHLY source dependent, and can sound GREAT with the right source. As a reference point, plugging them straight into an iPod just scrapes the surface of what these phones can do. Particularly in the bass department, the old Sony gave them some pretty decent slam completely missing from an iPod.
And the Sound Blaster 24 bit external soundcard is NOT the answer. Pretty much crap in my experience.
My advice is you get what you pay for, and I've tried several of the sub$100 external USB's and they were all pretty junky, or fried in a few months.