Sound Card Upgrade

Hello,

I have the Gigabyte GA-P35-DS3L motherboard, and for whatever reason, my
Stereo Mix will not work neither on Windows Vista or Windows 7. (Both
64-bit) It shows up under my recording devices, but even when it's selected
to record, there is not audio being recorded from my computer - it's just
silent. My sound also could be a little clearer, and my surround sound
doesn't seem to work too well.

Anyway, I'm interested and putting in a sound card, but I was wondering what
cards out there are good. I'm willing to spend around $100 (US) if that is
what it takes. The sound card will be for my Logitech Z-680 speakers. My
system does support PCI Express. I've heard some good things about
Creative's X-Fi Titanium and Fatal1ty cards, but are there better
alternatives for the same price? I would like a sound card with very good &
stable drivers with both Vista and 7 64-bit.

Thanks.

Other system specs:
Intel Q6600
8GB Corsair XMS2 DDR2 800MHz RAM (2GB x 4)
NVIDIA GeForce 8800GT 512MB GDDR3 PCIe 2.0
Seagate 500GB 7200RPM 32MB Cache SATA II Hard Drive
Western Digital 1TB 7200RPM 32MB Cache SATA II Hard Drive
Samsung 16x DVD burner (SATA)
Samsung 16x DVD burner with LightScribe (SATA)
Antec 650W Blue Power Supply
Xion Onyx case