A USB 2.0 card is definitely a must. While the card will unfortunately only will work as a USB 1.1 card, extra USB ports never hurt. Be sure to look for cards with an NEC chipset, as cards with those chipsets seem to be the ones that play best with Mac hardware, especially with sleep mode.
Another option would be an 802.11 PCI card. Later updates to Apple's AirPort software enabled third-party 802.11b/g PCI card support. On OS X, the cards are recognized as AirPort Extreme cards; however, under OS 9 they're recognized as AirPort cards running at 802.11b. Before, it was a real pain to figure out which cards worked and which didn't (you had to keep track of card revision numbers to see which cards did and didn't have the necessary Broadcom chipset) but if you do some searching around on eBay they're not too hard to find.
If you're interested in more exotic hardware, you can look around for the 5.1 version of M-Audio's Revolution PCI card, which had OS 9 drivers (that were a little dodgy at times), or Creative's controversial SoundBlaster Live! for Macintosh PCI card.
Another option would be an 802.11 PCI card. Later updates to Apple's AirPort software enabled third-party 802.11b/g PCI card support. On OS X, the cards are recognized as AirPort Extreme cards; however, under OS 9 they're recognized as AirPort cards running at 802.11b. Before, it was a real pain to figure out which cards worked and which didn't (you had to keep track of card revision numbers to see which cards did and didn't have the necessary Broadcom chipset) but if you do some searching around on eBay they're not too hard to find.
If you're interested in more exotic hardware, you can look around for the 5.1 version of M-Audio's Revolution PCI card, which had OS 9 drivers (that were a little dodgy at times), or Creative's controversial SoundBlaster Live! for Macintosh PCI card.