If your Marantz doesn't have optical, I'd recommend a UCA202 to your Marantz SR 480.
I like foobar player in ASIO or WASAPI mode.
A Xonar / X-Fi etc etc is probably way overkill
for you. You'd be better off using that money to buy better speakers or a sub.
At the cheap end of the audio spectrum (which is were you are), you will be speaker-limited, most electronics are 20-20khz +-0.5db with 80-90db SNR;
your speakers most certainly can't
do 20hz and even >16khz might be questionable, and you'd be lucky to get 50db SNR out of the speakers/room (say: peak SPL - room noisefloor A-weighted).
For speakers I'd recommend these here, or it's smaller bothers:
If you like it loud, they will easily beat any speaker south of $1700.
I heard a pair at jbrown's house last week paired with his 6000watt quad 18's; not as good sounding or as beefy as my system but for the price you can't do better:
We were powering it my $1300 DAC and $1000 pre-amp, and jbrown's $2500 worth of amps; so it's not really a fair comparison but it shows what $300 speakers can do.
Most speakers get even get loud enough to keep up with his 6000watt quad 18 subs; so the fusion 12's did very good for their price/size.
I use a hot-rodded 15inch version of which cost me $900 per speaker but it's easily comparable to my B&W 803's which were more than triple the cost.