It almost sounds like you need a pro audio interface. Are you looking for 8 inputs as well? There are a number of professional interfaces that use USB2 and/or Firewire, many with 8 descrete outputs, some with 8 descrete inputs as well. I actually use a Mark Of The Unicorn 828 audio interface, which is a firewire interface with 8 in/out. You can also check out devices M-Audio, PreSonus, Lexicon, Digidesign, Echo, Mackie, and a few others.
Your budget and needs will tell you what to buy, as you can find interfaces from $200 on up to $1K+ with various input/output arrangements. A couple things to think about though: Get a firewire device if possible. USB2 will work, but there's the possibility of added latency inherent to USB devices. A USB 1.1 will not handle the load.
If latency is a large consideration, make sure the device has ASIO drivers with it. WDM drivers will work, but the ASIO drivers are specificaly developed with a low 1-2 ms latency max in mind.
Finally, do some extra research in forums and the companies website if you also need to use the interface in Linux. I'm having this issue now, because the MOTU 828 I bought a few years ago is not supported under Linux at all due to their outright refusal to develop Linux drivers or release the specs so a third party could develop drivers for their firewire devices.