If I'm not mistaken, the beige G3 runs IDE hard drives and CD-ROM. That's really cool, IMHO, because that means you have a lot more options when it comes to upgrading the storage. Also, that means you should be able to add any off-the-shelf CD/DVD/burner to upgrade.
As for processor upgrades, you should be able to use any ZIF processor you can come across. One thing I like about this generation of Apple machines-- they're still decently expandable. I've got a PowerMac 8500, running with 400MB of RAM, and a 300mhz G3 processor. If I wanted, I think I could up this to a 1ghz G4....
***ADDENDUM***
Also, as far as PCI video cards are concerned: You have to get one specifically for a Mac. A lot of times the video BIOS is engineered for a Mac. Most other PCI cards should be okay, so long as you have a MacOS driver for the device. That means that in an OS like Linux, you ideally could use most PCI NICs, and maybe some sound cards, even SCSI cards and IDE controller cards.