Go with an ASUS A7V-333 for the motherboard. Asus posts the onboard sound drivers on their site, so that takes care of the sound/Motherboard.
I'd personally opt for an ATI RAGE card (any of the RAGE should be peachy) or a GeForce2/TNT2
Drivers in the latest SuSE and RH should cover either card.
NIC: 3C905B - Rock Solid 10/100 card, drivers have been in Windows in 98, and in Linux for longer.
Hard Drive, assuming you're not building this to run a mission critical 24x7x366 service, then any of the ATA drives should suite you to a T, stick to brands you know and trust, and buy as much space as you can afford. If Linux will be alone on the box, then you don't have to worry about boundries on the disk (if you try to drop a linux install on an existing drive waaaaayyyy out on the drive (past about 30GB in my exp, then you'll need to boot from a floppy)
Modem, stick with a REAL modem; check the box, make sure it doesn't say windows modem. USR/3Com once again on this one. I use a USR Data/Fax internal (it doesn't say windows anywhere). If you want to be 100% safe, go external.
Processor; if you go with the A7V, then you'll be looking into an Ahtlon XP processor. hit pricewatch.com to determine the price-breakpoint that you can live with, and get the fastest you're willing to pay for.
As for DVD/CD, I'm using a Pioneer DVD (read only) drive, and a Plextor burner, and I've been happy as a clam with both. I burn, I read DVD's no fits.
Fans/etc... Get fans that'll fit the fan spaces in your chassis, don't go trying to cram a 12cm fan into an 8cm hole. Look for fans with 3-pin molex connectors, so that you can connect them to the motherboard.
Now the stumper. The GATOS project on sourceforge has been working on getting all of the vivo features from the ATI All-In-Wonder cards to work under Linux/FreeX86. So far, on my box with an AIWRadeon 7500, everything they've produced works for me. http://gatos.sourceforge.net/
I've had mixed success getting a SoundBlaster Live! or Audigy working, but it's only been when I try to do digital output (to my digital speakers) that it hasn't worked. Otherwise, if you don't like the notion of onboard sound, the Live! and Audigy have worked well for me.
Cheers! and good luck