What is your budget?
Personally, getting a soundcard (M-Audiphile 2496) didn't make as much difference as getting an off-board DAC (Beresford), I don't think, but then I tested my system with and without the DAC several times, but didn't do that with the soundcard.
Soi far you seem a little, well, all over the shop. I like the AV amp idea too, in a way, but will you use it for anything else, and do you have room for it? How stuck will you be if you go way over budget?
A PC with neither soundcard nor external DAC will not sound terrible, just a bit less good than it otherwise might.