I just bought a G3 iMac on craigslist for $25. It's a nice computer, and it'll look awesome sitting on my Apple desk next to my eMac. It's a 400 MHz G3 with 64 MB RAM. It currently has OS 9.0.3 installed, which works, but I am a lot more familiar with OS X. With 64 MB RAM, would OS X (probably 10.0 or 10.1) run well, or would I have to add more RAM? And on that note, I've downloaded OS X 10.0, 10.1, and 10.2 from the FTP. Can I just burn the ISOs to CD and boot the iMac from the CD and install OS X? I've never dealt with an OS X installation on PPC before.

Just burning the ISOs to CD should work…

I would say 64 MB is way too little, I would recommend upgrading to 128-512 MB ram, as even with an older version 64 MB isn't going to get you far.

If the iMac has a tray-style ODD, then you'll need laptop SO-DIMM PC133, if it's a slotload, then you'll need desktop PC133, but I'd suggest a RAM upgrade as well.

I heard you can upgrade the ram up to 1 GB and the CPU up to 1.25 GHZ (I think).

Source:
Matt's Macintosh on YouTube.

(But since I have a bad memory, you better watch his review on the iMac G3.)

Sounds like you have the first slot loading iMac, a Powermac2,1. (It is the only 400mhz iMac that ran 9.0.3.)
Note: if I'm wrong on the model identifier some or all of what is below may be wrong.

Unfortunately, the only thing you can easily upgrade on that iMac is the RAM. It will handle up to 1GB (Apple officially supported only 512mb) of RAM and takes PC100 3.3v 168-pin SDRAM rated at 10ns or faster. This can be had for $50 or less depending on how long/hard you look.

The hard drive is a pain in the neck to upgrade on those iMacs (I've had to swap them out before) as you have to disassemble pretty much the entire computer all while being mindful of the electrostatic charge that can build up in the CRT. That said, your 400mhz iMac should have a pair FireWire 400 ports that would be usable if you needed a larger external HD.

As for OSs, the 400mhz officially supports up to 10.4.11. There is not presently any method for installing 10.5.x on any G3.

Drives,RAM and USB devices are interchangable with PCs.
To boot from CD,you hold C key.
You could even run older beta OS X versions available here.

It is indeed the first slot-loading iMac. I imagine I have some SDRAM laying around that I can install.

Update: Installed 384 MB SDRAM and OS X 10.2.

you could install the first Safari 1.xx releases on 10.2

Yep, I looked up what the latest versions of each browser I could install on 10.2. Looks like I'm limited to Safari 1.0.3, Firefox 2.0.0.20, or Opera 8.54. Of course, using OS 9 would limit me even further. I don't have it connected to the internet right now, however. I don't have ethernet cabling up here (at some point, I plan on getting a second wireless router to use as a repeater and then hardware a bunch of computers).

No, you could also use iCab 3.0.5; or if you install 10.3.9 you could use iCab 4.8.
http://icab.de/dl.php

laptop ram eh? i got some old spare laptops floating around, i remember opening up my imac g3 and seeing that it looked like laptop ram could fit however i think the ram i tried to put in it was too new and i didnt know about the 512mb rule so i failed lol im gunna give it a shot tho, i really hope your right lol

does anyone know what the maximum hd size can be?

Depends on what OS you plan to install on the system. I think the first gen iMacs (tray load) requires a 8-12GB partition for the OS and then any number of extra partitions after for data (something about the bootloader not being able to see a partition that is too big)
If it's a newer one, I think 128GB is the size of the largest OS partition, but it should be able to address basically any size drive. I have had an 80GB drive in my iMac.

If it installs, it would run terribly. 64MBs of RAM is nowhere near enough. Upgrade RAM first, then upgrade.

Is that a tray or slot loading iMac?

I'm pretty sure it's the original Slot-Sloading Model, Seeing it has a 400 MHz CPU (Since the fastest that shipped with a Tray-Loading Drive is 333 MHz) and he said it has Mac OS 9.0.3 installed

Correct. It's slot-loading.