Sound cards need to be SoundBlaster compatible. To make it simple, I just buy the real thing. I've used other brands, but it seems like there's always some program that doesn't have drivers for the other brand, so your best bet is just to stick with a SoundBlaster from Creative Labs. There are several models of SoundBlasters, but that all depends on how important sound is to you. Stay away from boards that combine sound and the modem, otherwise if you want to upgrade one, you have to upgrade the other also.

Sound cards were one of the last types of add-in cards to switch from PCI to IDE.  There are good PCI sound cards available now and soon IDE slots won't be available any more on PCs, so if you're in the market for a new sound card, it's best to look for a PCI version.

Other audio card manufacturers are Diamond, Ensoniq, and Turtle Beach.

Obviously it doesn't make any sense to buy the most expensive sound card and connect it up to $15 speakers.

Speakers

Speaking of speakers, make sure you get ones that run on AC. If you get battery-operated speakers, you'll be changing the batteries every month or so. There are a couple of manufacturers that make good speakers. One of the best is Cambridge Soundworks. Others that are worth considering are Altec Lansing and Labtec.

There's a new trend toward USB speakers.  There is an advantage to USB speakers in that you don't have to have a separate sound card, the speakers provide both the sound generation and the speakers.  The disadvantage to the USB speakers is that they are new, so many older games won't work with them.