The Performa 5200 series computers have both a communications slot and a PDS slot - both of which can accept networking cards. Although a communications slot ethernet card most likely will work, I'd recommend getting a PDS based networking card. They're generally cheaper and there seems to be fewer problems than with the Comm Slot cards.
Macworks.com has the PDS cards (they're called LC-PDS cards on the site), and they work fine. I have one running in my 5215CD - it's great.
Apparantly Farallon still sells the Comm Slot networking cards as well if you choose to go that route.