For a very comprehensive, and probably totally overblown answer, I'd suggest you go here and poke around.
http://acapella.harmony-central.com/forums/forumdisplay.php?s=&forumid=14
What I suggest for PC is that you get some kind of audio interface that comes with software. I use a Lexicon Omega, which you can get from
www.musiciansfriend.com for about $300. It includes an interface that allows you to record up to 4 tracks simultaneously using mic cable or 1/4" instrument cables, and also comes with recording software that you can record and mix 24 tracks with. It hooks to the computer via USB.
On top of the PC and the audio interface and software, you really need a decent set of headphones and to mix properly you need some kind of decent sounding set of speakers to listen on. For several years I used a shelf stereo system and that worked well enough.
You can just buy a $100 software package by Cakewalk or somebody and use the computer's internal soundcard, but there is a little bit latency when you do that, which means that stuff gets out of synch pretty bad sometimes. It drove me batty until I bought the audio interface.
As far as PC requirements, audio is huge resource hog. I'd suggest at least 1.5 ghz processor and about 1 gb or more of RAM. Also, you want a huge, fast hard drive. For a three minute song with 8 tracks, you can easily use a gb or more of hard disk space. My computer is a 2.6 ghz Pentium 4 with 1.5 gb RAM and 160 gb hard drive. I can usually get 16 stereo tracks with effects without bogging my computer down too much.
