I recommend an Asus Xonar.
Newegg.com - ASUS Xonar DX 7.1 Channels PCI Express x1 Interface Sound Card
And just FYI 8 channels is 7.1, not 8.1. I don't even think 8.1 exists.
I mean maybe an 8.1 speaker set exists or something.
7.1 is set up like this.
That's 7 satellites and 1 woofer, making 7.1
Where the **** would you even put the 8th satellite for 8.1? That just doesn't make sense to me. 7.1 is way more than enough already, and to the best of my knowledge a sound card doesn't exist that will drive 8.1.
Oh about the Asus Xonar I recommended. These things are absolutely wonderful.
As an example, I was using a nice Realtek 7.1 onboard sound to use FLstudio. When I got up to around 10 or 12 channels playing simultaneously the Realtek would choke hard. And by that I mean stuttering/skipping sound. This was even at a lowly 44.1khz @ 24bit. The Asus Xonar on the other hand took it to the next level and delivered 192Khz @ 24bit from the ASIO driver.
I can't begin to tell you how much better this sound card sounds.