I recently threw my old Audigy 2 zs back in my system. I have to say even with my modest sound setup(t-amp & $50 bookshelfs from newegg) there was a definite improvement.
I keep telling people that if they want a cheap soundcard, to pick up a used Audigy 2 ZS off ebay or something for around $30. One of the best purchases you can make if you have halfway decent speakers or headphones, and a good ear for detail.
Its an excellent improvement over onboard, and I'd even argue its superior to anything Creative is putting out NOW. Because that was when they had a HARDWARE approach (high quality DAC, DSP, opamps) instead of their SOFTWARE approach (X-fi, Crystallization, EAX emulation) to produce quality sound.
Any X-Fi chipset card should be avoided really, they are the weakest performers, have the most plaguing driver issues, and rely heavily on marketing gimmickry with their "f4tal1ty", "EAX", and "THX" nonsense..
The Asus XONAR models are pretty good, but nothing compares to any card with the CMI8788 chipset.. the holy grail of sound cards thanks to its glorious DAC.. They want $200 for the HT Omega cards with this chipset, but if you're lucky you can find a used Auzentech x-meridian, a very rare card, which has the same exact chipset. I was lucky enough to pick one up for $80 in mint condition.