I have found that most pc soundcards, internal or external usually have an output voltage in the 2v range, whereas a high end head unit will be at least 4v & some are as high as 8v. this makes a real difference when you are running multiple amps. & I don't think pc hardware manufactures ever consider this as a probability, (there's a business oportunity for someone inclined to do that) you can add line drivers to help remedy this, I have done one with an external usb soudblaster extigy & it worked well., the side benifit of a setup like this is that that particular soud card will also funtion stand alone, without the pc if necissary, & you could play a different source through it with the pc off & still have volume control through it among other features.
at this point in time I still think that an all out competion system is still better of being based on a hybrid system as oposed to a full pc front end, maybe this is just my opinion, but I have had a lot of experence with both pc car installs & competition in car audio, for whatever that's worth. check out this thread for more input on this:
http://www.mp3car.com/vbulletin/car-audio/35156-can-carputer-replace-high-end-av-dedicated-system.html