It really depends what you want to do with it.
If you want to rip video, play modern games or watch online video, as the previous poster said it's probably better to sell it and spend the cash to get something quicker (ideally with an intel processor). This is almost certainly a better idea than buying a new processor upgrade.
On the other hand, I have a 500mhz g4 to which I've added 2GB of RAM and 2TB of mirrored hard drive on a third party SATA PCI card and it works brilliantly as a home media server (to the apple tv and other macs in the house). I also use it for very occasional web browsing but nothing involving Flash.
It's robust, dead easy to work on, has Gigabit Ethernet and with remote desktop turned on I can manage it using VNC from anywhere in the house - can't ask for more.