OK, so if you've settled on redhat (personally, I found redhat too "business" and mandrake considerably more friendly - it's more aimed at the desktop) anyway, back to your questions.
1. How can I get a list if commands and there meaning/what they do for RedHat 9 ??
No/probably not, most of the lists of commands that are available tend to be stuff taken from places like O'Reilly publications - though you may find something here
- as your net access is via windows, then you'll probably have to get the PDF format doc's (and adobe acrobat, or some other PDF reader) and print them off.
If you check out the link's for "rute user" in my sig - I've heard lot's of people say it's an excellent guide (you can also get it as a hard copy book). Also, you've probably got the "man" pages installed as default - but they can seem very intimidating, so there's also a link in my sig about how you decypher them - As far as I know, all the commands have a man page.
Your quest for a book will only give you either the basic's or the specific's. A good example would be to look at the man page for "tar" (as in when installing from source)
then think about a "typical" command to unpack the "tar"ball, say something like
tar -zxvf /home/john/whatever.tar.gz
you've issued the tar command, but with 4 different "arguements" i.e. zxvf and if you look at the man page for tar and scroll down, I think it's under options, you'll see each of the letters zxvf is telling the tar command to do something different, and all that's before you've even told it where the file to unpack is, and what it's called i.e. /home/john/whatever plus you've confirmed for the tar command that it's a tar file that's packed up with the gzip compression facility hence the .gz ending
So imagine what you'd need to be able to decypher every conceivable command. You'd need a book on just the tar command. You wouldn't be buying just a book, you'd be getting a whole f*****g library
hence my pointing you towards the man pages and the link from my sig on how to decypher them. Or just try "googling" for man pages - there's lots of explanatory stuff there.
2. How can I access the list of sound cards which RedHat 9 will recognise ??
Because you're dealing with a VIA chip that is an AC97 device, it may not be doing just the sound, but that's the bit you want to configure - you may have to get the latest driver for it from the VIA site. You could try doing the command
as root in a terminal - you'd have to make sure that they system has the AC97_codec installed (my system -mandrake- installed it even though I've got my onboard sound disabled at the BIOS, and it's pointing at the driver for my soundblaster sound card), you'll see that from the "lsmod" command.
You might find a list of cards/devices, at the hardware compatibility list
section of the redhat site - but if it's anything like the mandrake hcl, it's not going to be as exhaustive as you might hope.
3. How can I access the list of Modems which will be recognised/detected by RedHat 9
Again, the hcl, linked above. But, because you're using an external modem, it may not be listed. If it's a "proper" modem, and not just some sort of winmodem
then if your usb system is seeing it, it should just work (theoretically - well, that's normally with serial/parallel devices). You'd just have to make sure that your usb system see's it. The only snag being that as with my scanner, because it's a usb model, it doesn't tend to show up with commands like lsmod and dmesg - it shows the whole system, but only the stuff "inside the box".
Sorry if all that sounds a little generalised - but to be fair, you've got a shit load of reading ahead of you.
I'll suggest something else though. I appreciate that you're looking into redhat 9 at the moment, but you may or may not have noticed, that Mandrake have released mandrake 10 community.
Don't even think about trying that. The identifyer of "community" is telling you that it's the testing version.
I've had my life made very easy for me with mandrake, by using boxed set's.
So my suggestion is that you wait, until Mandrake release mandrake 10 official. Then order a powerpack version. I promise you, it's worth the money. It'll be up to date, kde 3.2, a 2.6 kernel, ALSA sound, etc etc. But it should pretty well be install, and bingo, you're up and running (that's not guaranteed, but if you hard disc's are SATA type disc's then it should have full support for them - redhat 9 has been around a while etc etc).
In the mean time, keep plugging away. Or, if, like me, you're too impatient, then if you've got access to a burner, under windows, download (or you can probably order it from cheeplinux), the latest knoppix. That way, you can boot it, and see if it detects all the hardware (and knoppix, arguabley, has the best linux hardware detection available). If it see's everything, then you can install that to the hard drive. Google for the knoppix forum's, there's ton's of help, assistance, idea's and howto's.
And at least you won't have to wait to start using linux in anger.
p.s. If you do try using the lsmod and/or dmesg (as root, don't forget) and it seems that your usb system is being seen, then have a look into modem dialers (kppp comes to mind, that's in kde, not sure what's available under gnome - I don't use it) it might be a case of just telling your system where to look to see the modem.
And You also may want to see which sound facility/drivers your system is trying to use OSS (the older ones) or ALSA (the newer facility - which can be found if you google for ALSA)