Look at the link below.
Download the document "iMac G3 (summer 2001)".
First go through the first 40 pages to decide, whether you want to do it.
- You are shown RAM upgrade (2x 512MB PC100 or PC133 SDRAM),
- HDD upgrade (max. 128GB, if you want to add more, you need to install a firmware hack).
- optical drive (some drives require installing patchburn, a firmware hack. it is easier to connect a firewire (or USB1) external drive instead. you can also add a second HD, instead of opt. drive, but external firewire will be faster.
- airport card can be upgraded, but it will only give 11mbit, same as adding a USB-Dongle (USB1) which will give 11mbit, use the ethernet port instead for speeds of up to 100mbit.
http://home.earthlink.net/~strahm_s/manuals.html#imcs
If there is OS 9 or 10.2 installed, you will have to first install OS 9 on the new drive and download apples newest firmware for the iMac G3 DV. Then you can upgrade to Tiger (10.4).
Go to Ebay and search for the items you want to upgrade, then choose "ended auctions" from the left (you have to scroll down maybe a lot). Then decide, if you want to put that money in the machine. There is no way to get youtube videos play faster then 240p, so you might want to buy a newer Mac, if youtube-capability is the decision point for you.
There are more links explaining it in detail, but first decide, if you want to really do the upgrade. You may want to google for "iMac G3 upgrade" or "installing HD iMac G3" "installing hard drive iMac G3". There are several people that have done online guides to especially iMac upgrades.
Some will tell you, that you can add a IDE-SATA-adapter and use a SATA drive or a SSD...