DDR333 is DDR333 for the most part, so if what you've got is DDR333 it'll work together unless you have some kind of wacky Dell OEM board or something.
The hard drive is a good deal, yes.
And as for playing music in knoppix, you're mostly out of luck unless it already plays sounds. If it plays sounds, then it's detected your sound card, and all you need to do is run xmms or gxine or mplayer or whatever knoppix comes with these days (my bet it's xmms), navigate to your music folder*, and load music files. It generally doesn't work with playlists unfortunately.
If it doesn't play any sounds, then it hasn't detected your sound card, and you are unfortunately out of luck.
*Your music folder is probably on your hard drive, and you can access that by mounting it in your home directory in Knoppix. Don't know how to do that? Open a terminal window (usually called Xterm or RXVT) and run the following commands:
mkdir hd
mount /dev/hda1 ./hd -t ntfs --readonly
the /dev/hda1 should be replaced with your hard drive's device name. (the 'a' refers to it being the primary drive on the 1st IDE channel, and the 1 is the partition number, so the only partition on a hard drive that's the master device on the secondary IDE channel would be /dev/hdc1.)
The 'ntfs' assumes that your hard drive is formatted using NTFS. If it's FAT32, then replace the ntfs above with vfat.