Find (Apple-F) System Profiler and run that. In OS X 10.0 (Cheetah) it still looks like OS 9's System Profiler, but it has CPU information.
The first generation tray-loading CD-ROM iMacs could take up to 512 megs (256 meg 66 MHz or faster SDRAM SO-DIMM), and the later iMacs could take 1 gig (standard PC-100 or 133 MHz SDRAM DIMM). You can run up to Mac OS X 10.4.whatever on any G3, but if your iMac doesn't have Firewire then you can't
install it on that machine - you have to install it onto the hard drive using another machine and move the drive.
Hard drives of any size can be used, but drives larger than 8 gig on tray loading should have a small partition (less than 8 gigs) at the beginning on the drive with Mac OS 9 on it from which you can run Xpostfacto to boot OS X from the rest of the drive. Drives larger than 128 gigs need all the boot partitions to be below the 128 gig limit so that the OS can load SpeedTools ATA Hi-Capacity Driver if you want to use the rest of the drive.
I have a first generation iMac which I've built into a Tonka truck:
http://www.ziaspace.com/tonka/installation.jpghttp://www.ziaspace.com/tonka/tonkaonshelf.jpgIt runs Mac OS X Server 10.4, has a Sonnet Hamoni 600 MHz G3 with Firewire, 512 megs, a DVD burner, and a 2TB SATA hard drive on a SATA-IDE adapter.
I don't know why people irrationally connect old with unstable, but unless your battery is dead or your hard drive is dying, there's no reason to think it wouldn't be stable. Mine has uptimes of many months at a time.
As far as speed is concerned, who cares about whether it can run Flash? Install ClickToFlash and your CPU won't get pegged from every advertisement. Flash is too slow on the fastest machines, anyway.
Mac OS X is pretty fast on modest machines. It runs very well on older PowerPC and even on low end Atom systems.
