I've tried more cards than I care to remember as I've been doing audio PC for at least 5+ years now. Even the ultra expensive internal pro cards are not as good via analog output as the alternatives. External interfaces like those from MAudio, RME, etc. try to bridge the gap by getting the delicate stuff on a soundcard outside the noisy environment of the PC.
In my opinion, an Audio PC should be used as a transport to feed music digitally to an external DAC. Trying to do analog out from inside the PC on a sound card is not going to achieve the best possible audiophile sound.
The perk of an audio PC over some of the other alternatives being discussed here (Sonos, Logitech's products based on Slim Server, etc.) is that it's a PC and can do all the other things a PC can do while you are listening to music. For those of us with the attention spans of a gnat (I'm raising my hand here) you can surf the web, respond to customer emails, do video editing, etc. all at the same time that music is playing since its right on the screen there with the music. All from one device. Streaming out digital audio via Foobar, MediaMonkey, etc. is not CPU intensive at all. You don't need a lot of resources to do that. So if going the USB method, you can use a rinky dink PC and be fine. It can be easily done with the ~$120 Linux based fanless all in one board units to $240 Dell desktops, etc. For what I'm doing, the only caveat is that you need a spare PCI full height slot in the case.
You can get an audio PC pretty darn quiet. Windows 7 also has some great power saving features built in. I finally got around to adjusting my bios to control fan speed, put an SSD drive in my machine (no hard drive noise!!) and you can barely hear the PC when you are within a foot of it and everything else is turned off in the house. At the listening seat, nothing...
For getting digital audio out of the PC, my personal choice is a Lynx sound card with custom dongle to replace the included breakout cable. I tested it against an asyncrhonous USB to SPDIF converter and I preferred this method. The Lynx AES16 is the standard amongst computer audiophiles as far as I can tell for folks trying to get AES/EBU out of their audio PCs. The L22 can do the same thing but is usually more expensive because of it's DAC capability for analog output.