Why would you want to add a PCI Ethernet card to a Power Mac? Although you can take advantage of some tweaky multi-homing possibilities with multiple Ethernet cards, in my case it was much simpler: a lightning strike near our house fried my Power Mac G4's onboard Ethernet!
I actually saw the lightning hit an ash tree on the other side of our driveway, or rather, I heard the simultaneous blast of thunder and saw a chunk of bark blown off the tree. All our uninterruptible power supplies screeched in unison, but everything seemed to continue running. When I investigated, though, my network had completely lost its mind, and I had to toggle power for every device on the network to restore functionality.
Only one Mac didn't recover fully: my dual 1 GHz Power Mac G4. It booted fine, but Panther's Network preference pane kept reporting that the cable wasn't plugged in to the built-in Ethernet port. Fiddling with the cable and the 10/100Base-T switch made no difference - clearly its little Ethernet chip soul had floated off into the ether. I turned on its AirPort card and continued with my day at 11 Mbps.
It wasn't long before I realized that I'd become addicted to the 100 Mbps throughput of 100Base-T Ethernet though, and working with files on our similarly equipped server was now painful. Backups took forever as well, and I decided that I had to buy a new PCI Ethernet card.
Ambiguous Research -- Although Small Dog offered several PCI Ethernet cards for sale for entirely reasonable prices, some quick research showed that they needed drivers to operate correctly. A comment on a list of smart Mac friends led me to believe that some PCI Ethernet cards used the same chipset as Apple did for onboard Ethernet and thus wouldn't require additional drivers. Normally I wouldn't care about drivers, but I didn't want to be forced to hold off on upgrading to Mac OS X 10.4 Tiger sometime next year just because my Ethernet drivers hadn't been upgraded.
My friends encouraged me to look for cards with the DEC 21140 chipset, since Apple's built-in Ethernet drivers support it directly, and poking around with Google revealed that a Linksys Ethernet card used that chipset. Unfortunately, after I purchased said card, I discovered that version 4.1 of the card may have used the DEC chipset, but version 5.1 used something else entirely. My Power Mac didn't even notice when I plugged the Linksys card in. That's when I came to understand the impossibility of figuring out exactly what chipset any particular card had without being able to see the actual card in person.
I had also come across a page on the Accelerate Your Mac site that implied an Intel Pro/100 Ethernet card would also work without drivers. The report was minimal, so I'd been leery to start with the Intel card, but I decided my chances of the Intel card working were better than guessing at another card that might use the DEC chipset. Needless to say, Intel said nothing about Mac compatibility on the product page.
A quick price comparison on NexTag turned up a number of vendors, and although none of them claimed Mac compatibility either, I took a plunge and ordered from Page Computer, since I'd bought supplies from them successfully before.
Long Story Short -- The card arrived a few days later, I unboxed it, shut down my Power Mac G4, installed the card, and restarted the Mac. When I opened the Network preference pane, it threw up a prompt telling me that it had found a new network port. I configured the new port with the appropriate TCP/IP settings and it's been working fine ever since.
The moral of the story is that if you need a PCI Ethernet card for a Power Mac and don't wish to mess with third-party drivers, the Intel Pro/100 M card may be your best option... unless Intel decides to modify their chipset in such a way that Apple's drivers no longer recognize it. So be sure you can return anything you buy.