There's nothing to stop you adding multiple tuner cards to your PC, other than available slot space. There are cards out there that are themselves dual tuners. Then you probably need some PVR software. There's a few options out there depending on your OS. I'm using Mediaportal right now, I don't particularly like it or dislike it. Depending on where you live, it can pull in electronic program guides (as can most of the other products).
Recording doesn't take much CPU power, but you do want a nice fast hard drive if you're recording more than a couple of programs at once.
If the recorded format is MPEG2, there's again plenty of software that can edit it, including what comes free with Windows. I've found that sometimes recorded files can contain glitches that make editing software fall over. Therefore I use a cheap prog called VideoReDo: at the simplest level, it can simply fix glitches. But it's also a very fast, if simple, editor giving you frame-by-frame control. Finally, it does advertisement detection. There are plenty of alternatives out there.
One of the many PVR forums would probably have better info.