Hey ther Maz, I will take a stab at a few of these issues. But wait, don't be afraid to dig in some of the other archives here if you get the chance. Some few of your questions have been covered by some real sharp dudes.
The PC: I use a PII 433MHz and find it is fast enough for me. But with the blazing speed of new chips out there you can probably get a 500MHz cheap. 32MB of ram is honestly a bit weak. Most software for recording tends to suggest 64 minimum and 128 reccomended. Bottom line the more the better as far as RAM goes. Same with the Hard disk. 4.3 seems large at first glance but I assure you that it will get filled in a flash. 10 gigs is pretty standard. Just to ballpark the PC deal for you, in todays little circuit city flyer you can nab a Celeron 500MHz, 64MB ram 10 gig machine for $900 ( I added the 400 they subtract for the damn compuserve deal). And I am sure if you hunt you can do better even still.
Soundcard: you want to record a track while playing back a seperate one. For that you will need what is called a full-duplex sound card. Ask the PC dude, many stock sound cards today are indeed full duplex.
Tracks vs channels: not litterally the same thing. A channel for example is the line in. Thats to say a 16 channel mixer or soundboard will accept up to 16 inputs or lines in (mics, instruments). But a track is the referance to a recorded voice (or part). Example:record the rhythm guitar and you have one track. Play it back and record the solo... second track.
Preamps: Generally speaking you will want a preamp for the mic, its true. The pre amp "conditions" the signal from the mic. Without it your sound frequencies go all over the damn place. The cost of these buggers is like anything else. but range from like $100 for a cute little ART tube single channel, $300 for a Duel channel (for 2 mics) on up to a ga-zillion dollars for a joemeek preamp (actually around $600).
Software: What you want is a multitracking software program. Both Cakewalk and N-tracks are that. Yes they let you playback and record at the same time (assuming you got the full duplex or interface sondcard) But a word of warning with cakewalk. It is close to useless if you are not midi ready. I vote N-tracks to start with. A for cost, B for ease. But both Cake and N-tracks are happy to accept effects.
Cost: Hmmm. We covered a lot of alternate options here so it is going to be hard to tag it depending on how carried away you get.
PC: lets just kiss $1000 off the top and assume it has a workable sound card you are ok with at first. If not .....
Decent sound card interface: sheesh I don't know, they go from $100 for a good card on up to $1000 for the latest and greatest MOTU or AArk deals. Maybe we can answer that one more so after you get your bearings?
Pre amps: again $100-300 for the most part. Oh you asked about Mics. For general purposes and not breaking the bank you can pick up
a shure SM58 for a hundred a piece...good for vocals, and
the Shure SM57 for a few bucks less is good for amps and drums.
Keyboards: sheesh. I vote you hit E-bay or a swap shop and nab a used machine if you are just hoping for midi control and some mild sequencing. I nabbed
a roland D-10 for $250 bucks and it is doing me just fine.
Dude I can't type anymore. If word gets out I wrote this much my folks will want a damn E-mail or something.
Jump in guys and clear up any stray path I may be leading this dude down. I am the first to admit there are at least 20 regulars here that kow way more than me.
