I'm using an HT Omega "Striker 7.1" PCI sound card with Windows 7 64 bit. Works great and has the System Mixer function. Actually, I had a newer SoundBlaster Xfi installed but had problems and the Striker 7.1, that I pulled out of my old Dell Dimension E510, works better than the SoundBlaster.
It's available at newegg here Newegg.com - HT | OMEGA STRIKER 7.1 Channels PCI Interface Sound Card
One issue, and that is with ANY add on sound card. You will most likely lose the front panel headphone and mic jack capability. Dell uses proprietary (and undocumented) front panel connections on many systems (I do a lot of support on the Dell desktop forum).
If you buy the Striker 7.1 you will probably have to download the Win 7 64 bit driver from their web site. http://www.htomega.com/striker.html