I just tried a back to back with comparison with the o2 combo and the Fiio combo using the m80s...  I have to say they both sound pretty much the same.  Maybe abit more instrument separation with the o2 but all in all the e17 sounds just as good as the o2.   However when I use my AKG Q701...

Just got the objective 2 + odac combo.  Holy crap. sound quality makes the Fiio e17+ e09k sound like a cheap toy... and I was already impressed with the Fiio combo.   I never really heard that much of a sound quality improvement on my m80 with the Fiio... however with the o2 combo friggin...

I have the e17 and the X-fi and I agree it would definitely be a big improvement over onboard sound, however the X-fi isnt too bad. Does it sound better than the x-fi... YES.   Is it a $150 improvement? absolutely not.   If your interested I'm receiving a jds labs O2+ODAC Combo next...

Judging by the components you have you honestly dont need an amp+dac unless your PC is garbage.  Part of the reason I bought the m80s is because they dont require an amp.  Seriously I would save the cash on your future headphone upgrade and a more dedicated amp + dac.