That all depends, you can get reasonable video cards for these to fit into the PCI slot that will enable quartz extreme add a G4 CPU and you can go all the way to Leopard on these machines with some hacking. It just depends how much money you want to throw at it. You have PCI slots but they're only 32bit wide so the fastest thing you can run on them is Firewire2 or Gigabit ethernet for data transfer, or internally SATA1 bus speeds.
I wouldn't bother upgrading the internal video RAM unless you want to stay with OS X 10.4 and below as Quartz and core image wont be supported by the internal video card and its only really a crappy ATI Rage chipset anyway. While with 6mb of RAM you can play period correct games, and I've played the sims 1 which is fairly 3D intensive on it, it's really nothing to right home about and you should populate one of the PCI slots with a Radeon 9200, or above in PCI mounting format if you intend to do anything intensive with the machine.
If you do upgrade your video, due to the limited amount of slots you have try to get a video card that is either passively cooled and doesn't require a large heatsink that covers a PCI slot or go for one which only covers a single PCI slot. You'll want to use your other two PCI slots for USB2 and SATA as well as or maybe including a card that has external sata.
The best part of these machines is the cheap and plentiful SDRAM and IDE with 256mb chips you can go all the way to 768mb for around $20-$30. Being an old world ROM machine you will run into hard drive size limitations via the IDE bus for a total of 128GB without driver support, although I'd only be using the internal IDE for optical drives at this point and would fit a SATA controller ideally.
You will want the full 768MB of RAM if you intend on using one of these machines with Leopard on a regular basis. You want as much RAM as you can get particularly given the slow 66mhz system bus speed in these machines.
You can fit an SSD, but without TRIM support you'll want one with good internal garbage cleanup processes and then you might have issues anyway, because not all SATA3 drives like to run at SATA1 although there are cheap and plentiful drives like the Corsair Force3 that will operate at SATA1 happily in a Mac.
You can and I have achieved modern Wireless N on a PPC mac by using an inexpensive USB2 to Wireless N adapter dongle to get it safely and operationally onto a modern wireless network. That's quite straight forward if you need to bring your PowerMac into a modern wireless N network.
To get front mount USB and/or firewire look for any standard 5.25" bay adapter and be prepared to give up one of the 5.25" drive slots of which you only have two on these machines if its a desktop or three if its a tower. It's really better to populate one of your rear PCI slots with rear mounted USB, or perhaps a combination card that also contains firewire.
I wouldn't worry about fitting an internal airport card at this stage as they're all too old and don't have the necessary wireless encryption to secure a modern wireless network.
