how do i get the best audio quality with the hercules gametheatrexp 6.1 and fb2k?
is fb2k any better than winamp?
and anyone knows how to make use of the hardware mp3 decoding in my card?

I use these settings: Master Volume at ~55% (test with udial.wav at low volume, it sounds worse above that), Mixer WAV, CD, Synth volumes at 100%, rest muted. Equalizer off, MP3 hardware playback off. Foobar output DirectSound, not allowing hardware mixing (again from udial testing).

i thought hardware playback would be better than software?
wads the diff between directsound and directsound2.0?
for the gtxp leave 3d audio acceleration at maximum?

Main difference between DS and DS 2.0 is fading.
According to this document:
http://oem.hercules.com/downloads/GameThea...roductSheet.pdf
set your output depth to 16bit fixed point. You can also enable dithering.

enable strong ATH noise shaping dithering?

Yes.
The 'strong' word here is misleading, don't worry. It is the recommended dither method, as you see from the description.