Re: which sound card
I'd suggest the Asus Xonar too. I've heard some good things about it.
X-FI can be a bit hit and miss, and I know. I have one.
The Auzentech version listed on your third link however is a better version than Creative provides, if what I've been hearing is correct.
My understanding though, is if you play games, the X-FI Prelude will give you full EAX and OpenAL over digital 5.1, which the Asus Xonar can not do, but in real terms, does this really make much difference.
Both cards will sound amazing though with a digital connection to speakers.
Abit IP35 Pro / Q6600 G0 / Zalman 9700 / 8800GTS 640mb / 4x 2GB Corsair XMS / X-FI Xtreme M / 1x 1TB / Antec 900 / Logitech Z-5500 / Samsung 20inch