Processor upgrades are available for Tray-Loading iMac G3s. These models have the processor on a daughtercard, but the later slot-loading models have it directly soldered in, making upgrades virtually impossible unless you are skilled in BGA rework. FastMac offered an "upgrade card" for these later models, but as you would probably expect, it required a complete motherboard replacement. This upgrade gave you a 550MHz G4. Good luck finding one today though! Most upgrades for the iMac G3 were available for the iMac G3 Tray-Loaders, and were made by Sonnet, Newer Technologies, PowerLogix and FastMac. The fastest G4 upgrade for these models was made by PowerLogix, with their iForce G4 500 clocked at, you guessed it, 500MHz. The fastest upgrade by clock speed alone was the Sonnet HARMONi G3/600 FireWire, which not only included a 600MHz G3, but also added a FireWire 400 port!
Here's a link to a page with more info on iMac G3 processor upgradesAll iMac G3s use 3.5" IDE/PATA hard disk drives. To install an SSD inside of them, you would need an adapter to convert the interface to a CompactFlash card, SD card, or even a proper mSata SSD! I would recommend going with the mSata adapter if you are planning on running OS X on the iMac. Yes, it's overkill, but CF and SD cards can really be bogged down by the newer operating system. If you're just planning to run OS 8 or OS 9, you should be fine with a storage card instead of a proper SSD. You may need a second adapter to convert from 2.5" IDE to 3.5" IDE.
The Bondi Blue iMac G3s had a "secret" expansion port, known as the Mezzanine slot. This slot was only present in Rev A and B iMacs, and Apple claimed it was completely unsupported. Still, a couple upgrades were made to take advantage of this forbidden slot.
This card enabled Voodoo2 graphics and 8MB of VRAM. It would have been useful for users who wanted better gamin performance on their iMac.
The iPro RAID was an upgrade card produced by Formac that added SCSI to the G3. It used a Slim SCSI 3 connector due to the limited space available in the iMac.
Pretty much the same as the last upgrade, but it added a TV tuner and S-Video output in addition to the SCSI port.
So as you can see, despite the limited upgradabilty of the iMac G3, many companies still managed to squeeze a lot out of these beautiful machines. Good luck finding any of them today though, they're all very rare! So, if you have one of these upgrades, enjoy having one of the rarest upgrades for Apple computers!