Well, you can upgrade from the 250MHz 603 processor to a 500MHz G3 Sonnet Crescendo card (I have the 400MHz). This makes a massive difference, and is a card in the L2 slot in the rear. You would need drivers and the fat back though.
RAM is usually at 32MB, so bumping up to 128MB is easy. I have noticed that the main unit won't come on for up to 20 seconds with more RAM in it, but it is handy to have the RAM.
The hard drive can be swapped out with a CF adapter ($1.71 incl postage from China), and a CF card ($25 for a Kingston from the UK), this results in quicker read/write speeds and the security of the data - no IDE spinny drive to fail on you! Easy to backup too.
If you find a USB 1.1 card with the Opti chip, you can add USB capability as well.
All of the above is easy to add and just as easy to remove, the hard part is getting some of the parts. If you really want to do it, you can. And I'm sure the more tech savvy guys here can recommend a host of other upgrades, which I eagerly anticipate!
Does this answer your question?