Old 700 MHz G3 iMac + Tiger = good luck getting any games to run on it. For OS X games, you hardly have any oomph. For System 8/9 games, you have solid specs but the wrong OS.
Do you have the HDD space to dual-boot? If so, there are lots of old-but-good System 9 games you can run (Myth, MWII, Total Annihilation, Duke Nukem 3D, Diablo).
Blizzard games - Diablo II & Starcraft - should/could run OK on both your specs and OS X. Some version of Civilization III - maybe the original, NOT 2006's Civ3 Complete - might also work. Myth II should work with the OS X patch/update. Ambrosia's Escape Velocity Nova is a bit of a rip-off, in my opinion, but should work OK given your setup.
But in general, if you're serious about gaming, I'd say set up a dual-boot; or take OS X off your iMac; or spend $25 or so online to get a System 9 iMac or G3 desktop & use that for gaming. If you're serious about OS X gaming (cheaply), drop some money into a G4 or Intel Mini.