By "can't log on to the desktop", what exactly happens? Does your X11 (graphical display) just die upon entrance, or does it give you a message? If so, what's the message? It might also help if you post the "Screens" section of your /etc/X11/XF86Config file. Looks something like this;
Device "NVIDIA GeForce 2 MX (generic)"
Modes "1024x768" "800x600" "640x480"
That's mine and it works quite well for me. As for the nvidia drivers, yes you are correct. You can get them from nvidia.com as well as installation instructions (there's a section in the huge README about installation instructions for the impatient. Follow those, edit your XF86Config file accordingly (we'll help if you need it), and you'll be styling). A word of advice though...don't get the latest drivers. I've tried them, and so have a LOT of other people. Apparently they give you incredible performance, but they're buggy as hell. I swear to God they basically said "Welp...it compiles. Ship it!" Get the second-latest drivers, which have been nothing but good to me on my RH8 system.
RE: Your internet. You have a cable modem you say? You wouldn't happen to know whether it's pppoe or not, would you (ie. do you have to connect to the internet or are you always connected)? One easy way to find out. Open up a term, become root user (command "su", and type "dhcpcd." If it waits a second or two and then dumps you back to the command prompt, try opening up a browser. Might work. If you do use pppoe, try Roaring Penguin (http://www.roaringpenguin.com/). If dhcpcd isn't present on your system (it's not on mine), I belive it's downloadable. You clearly have internet access as you've posted to a msg board, so do a google on dhcpcd. Copy the util to a floppy or cd, and grab it from linux. If you copy it to a floppy, you can access the floppy by typing "mount /mnt/floppy" in linux and treating the /mnt/floppy directory just like another part of your file system. Hope this works for you, as internet really is key to learning linux.
As for your sound, well I really suck big floppy donkey dick at troubleshooting sound so I'll leave that up to someone else. A few questions to get them started. If you put a cd in the drive, your desktop cd player probably pops up. Any sound? Is there any mention of anything to do with your soundcard when you become root user and type "/sbin/lsmod"? In fact, try typing "/sbin/modprobe/snd-cs46xx" as root. See if that doesn't help a bit.
Anyway, good luck getting these things solved. Hope this helped a bit.