For WiFi-there is a Motorola PCI card that uses the same chipset as the Airport Extreme cards used in later G4s and all G5s. Under 10.4.11, it gives you the same functionality, which means 802.11g speeds and support for WPA2 with some more modern encryption(I can still connected to the enterprise networks at work with these cards). As anyone who has ever done aftermarket upgrades on Macs know, these sort of things are often either plug and play with no set-up required, or you're in for a world of hurt making them work. This particular Motorola card falls into the former category, and if you plug it into a PCI slot, it's recognized natively as an Airport Extreme card with no other set up other than connecting to your WiFi network.
I don't have the Motorola p/n handy for this particular card, but I have a bunch of them around and have been installing them in G4s in particular for years. At one time, I was paying $10-20 each for them on Ebay. I haven't bought one, or even gone looking in several years, which is why I don't have it handy.
For a G4 upgrade-bear in mind that you will need a firmware tool to "unlock" G4 support in the B&W. Interestingly enough, the as-shipped firmware supported G4s, but one of the OS 9 upgrades(either 9.1 or 9.2.1) requires/will install a firmware upgrade that locks out G4 support. A few years ago, I acquired one still running its original OS 8.5.5 install, which in retrospect I should have left alone(or at least dumped the firmware)-in upgrading to 9.2.2 I had to upgrade the firmware.
Also, I suggest avoiding the plentiful and inexpensive Yikes! sourced G4s, which in my experience can be buggy. Sonnets in the 500mhz range are often inexpensive, and tend to be plug and play. Many CPU ugprades will require you to play with the jumpers on the LoBo, which are were shipped in a solid block under a big "warranty void if removed" sticker. Some of these jumpers control the bus speed, which you don't really want to mess with unless you have a good reason(some CPU upgrades, like the 800mhz+ Sonnets, require you to). You can do some things like mild overclocking, however, even with the stock CPU by changing jumpers around. The jumpers used to set things like master-slave on IDE drives are too big-you need small ones, which are sometimes called "SCSI jumpers". At one time, I bought a bunch for a little of nothing from China-I need to buy more as I've used and given away enough that I'm nearly out.