There are quite a few cards out there that will record 8 or more input sources at once. MOTU has a good reputation and a modular system where three different units can be plugged into a single card. You can easily expand it one piece at a time. Layla owners are pretty loyal, and I've heard great things recorded with 'em. I recently bought a Gadget Labs 8/24, and have few complaints so far... but it's new and I haven't pushed it too hard yet. That's just 3, there are oodles of other good cards with various pros and cons.
As for how the audio data is managed, that's a function of the software you end up using.
I like to suggest getting a reasonably inexpensive sound card to start with and get your feet wet. You might out-grow it fairly quickly, but you'll have a much better idea of what you need when you decide to plunk down a larger chunk of change.
As for the "more serious" cards out there, I'd really read up, ask around, and know the limitations of what you're getting before you buy. It's tough, because limitations are not advertised nor will they be brought up too often by salespeople. You're probably not gonna want to be upgrading cards every 6 months... unless you like losing your hair. The payoffs are sweet with PC recording, but it can be a technical mine-field. Some cards don't behave with some chips or some software. Heck, it can be hard enough some times even with good planning. Make sure you plot your goals and take your time with your purchases. You'll thank yourself in the long run.