I have an X-fi Fatal1ty edition, that i use with my:
Sennheiser HD555's via front heaphone jack on X-fi IO
OR
NAD C352 with Richter Wizards speakers and Richter Krakatoa via RCA out analogue
OR
Denon AVR-1804 with Richter Wizards, rear and centre. with Richter Thor Sub via digital coax off from X-fi IO
I use the heaphones for mainly quiet game play or music (mum and dads room is upstairs and dont appreciate the sub drone). The NAD is used for general music listening and some games (mainly C&C3). And the Denon solution is used for more speakers on music, or movie surround sound.
I believe the biggest jump in audio quality will always be speakers and amps, although onboard to high end discrete sound cards make a noticable difference if your on a budget your best with better speakers then highend sound card hardware.
JasonPCTec h: Why do you not use the Crystaliser? I love it, seems to make music out put through any speaker sound crisper and cleaner. Just my opinion though
EDIT: By the way Richter are Australian owned and constructed, so you guys might not have heard from them
www.richter.com.au