If you're using a WinTel computer, I'm not sure what options you have for balanced surround out. Maybe one of the higher-end M-Audio boxes? I've never tried hooking up my Mac to more than 2 speakers, but the Audio MIDI Setup utility recognized that my MOTU 828 and 2408 each have 8 analog outs and can arrange them in a 7.1 layout. Someday I'll have time to kill and give it a shot. Rest assured that I'll let everyone here at Audioholics know what comes of it.
I also have no experience decoding external surround digital audio streams with a computer, although if you have an optical input it should be possible. Probably just a matter of finding the right software.
A couple other pre/pros you might want to check out: Parasound Halo C-series, Krell, and Classe. Unfortunately, they're all on the spendier side of things. A question: If you're using your computer to do surround processing and input switching, why exactly do you need an external pre/pro?