Here's another option to consider your ASUS P5KC 192Khz/24bit output and 8.1 sound. That's pretty damn good already. It's probably your Altec system that's holding you back even though they are pretty fine.
I popped for an Audigy2 on my P4 board despite having onboard sound a few years back just to get that extra few FPS in my gaming and I regretted it. I should have put the extra $120 towards the Logitech Z680 5.1s that I bought.
Although the Altec lansings are great, sell them for $60 and get yourself a set of these:
Logitech Z-5500 THX Certified 5.1 500-watt Dolby Digital & DTS Hardware Decoding Speaker System 970115-0403 in Canada at TigerDirect.ca
The Z-5500 are the successors to the Z680 and are also THX rated. Believe me these babies shake your dishes when cranked.