For a sound card, get a real Sound Blaster card from Creative Labs, not a cheap clone like Compaq or other OEM's like to make. Creative Labs is pretty well supported. It's like buying a 3Com ethernet card, but be sure you buy a compatible model. The newest models may not have driver support quite yet so bring a print-out listing of the sound cards which your OS supports. If it is listed you can feel good knowing that you will be able to get your card working right away.
Video cards are much simpler.
A normal installation of your system will not need advanced graphics so any video card will work, but once you want to start using X Windows you will want a supported card. The number of supported cards is quite large, so odds are the card you choose will work, but be sure to check the supported list at the XFree86 web site. I have noticed they generally have well over 2 megs these days. Get a card with 8 megs if they have it. That will affect the possible resolution and color depth for X Windows.