Personally, I think the sound quality is better when you use analog outs on the Creative cards. Switching to digital, to me, makes the sound slightly less punchy. So you should try the switch and see if you like it, the cables are cheap. It won't be a big difference.
Auzentech and Asus both have some great cards. On paper they have better specs than the creative cards, but I'm not sure you will hear a big difference other than slight bias at certain freqs. This is kind of standard with any av equipment, and why people are partial to some brands over others.
Some of the cards have specific features that may interest you, such as hdmi out, dolby digital live, and the ability to bitstream certain formats over the spdif cables.
Here's a page from newegg with some potentials:http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100008658%20600011892%20600011982&IsNodeId=1&bop=And&Order=PRICED&PageSize=20
Bottom line: if you want to blow money go for it, but don't expect it to sound a lot different. The card you have is pretty nice still.