!
Whooo, System Shock! I'll be fair about the iMac, in regards to 3D gaming - it ain't great. The original run of the iMac had either 4MB or 6MB (yes, 6) of video memory, which I think is barely enough for the 'Shock. While the iMac should *slay* the three Wing Commander games, I know for a fact it gets pokey with both Quake III and Rainbow Six.
Granted, those two games are a little more recent than System Shock, but I wouldn't expect the world in regards to the iMac's 3D capability.
Let me steer you here, LOAF -
http://www.everymac.com/systems/apple/imac/stats/imac_333.html
This site is an excellent resource for older Macs! And no, I don't work for them / troll for them / anything like that, but I have used them as a resource several times in the past while buying an OS 9 - era Mac. The page I linked to has stats for the old iMac, but links on the left side of the page will give you info on G3 Powerbooks and Powermacs, as well. G3 Powermacs (the blue and white towers - remember that funky color combo???) support OS 9, and are much more powerful than the iMac. Heck, I think the first batch of G4 Powermacs support OS 9, too!
The last thing to keep in mind (and I apologize for going on a rant here, but I'm honestly trying to help ^_^ ) is the version of OS 9 listed with each model of computer. EveryMac will list the original OS with each machine it describes, as well as the maximum OS. Make sure the machine you buy lists its default OS as something prior to OS 9.2.2. Anything machine that came with OS 9.2.1 or earlier can accept a clean install of OS 9 via retail install discs. In other words, you can pick up an old retail copy of OS 9 and install it, cleanly, on any Mac from 9.2.1 on back. Macs that list 9.2.2 as the default had both OS 9 and OS X installed from the factory, and *require* the original system restore discs to allow OS 9 to be re-installed. These discs are a pain to find, believe me - I've thrown away hundreds on ebay looking for discs to match systems I've owned, and they only work properly about a quarter of the time.
For me, picking up a retail copy of OS 9.2.1 was one of the wisest decisions I've ever made, computer-wise. I can't tell you how many times I've zapped (and re-zapped!) the old iMac hard drive with it, and it works every time. It makes me long for the good ol' days where backing up and formatting a hard drive wasn't a total nightmare, you know?
Anyway, I hope all the info helps! Good luck machine shopping, and I hope you find you dream model soon!