You've probably figured out already that most new games want a much newer processor. I've run into that too. You're going to be shopping the used market.
How much VRAM does the nVidea 6800 have? Should bbe plenty.
Depends on the kind of games you like. Over the years my son and I have shared the "Dark Forces/Jedi Knight" franchise from the Star Wars Universe. Lucas Arts skipped the Mac platform with with the original Jedi Knight and returned to it with "Jedi Knight 2: Jedi Outcast" and later, "Jedi Knight: Jedi Academy." The later is the more demanding but runs fine on my G4 1.25 single processor. There is occasionally an outside scene that get a little choppy but it's not a dealbreaker.
As for WW2 games, the earlier "Medal of Honor: Allied Assault" and MOH "Spearhead" run fine. Later versions of the MOH franchise may need more computer.
One of my favorites is the "No One Lives Forever" pair. All tongue-in-cheek, poking fun at 1960's spy movies that combined some puzzle solving with a FPS. The first version ran OK on a G3/500 PowerMac; NOLF2 needs a G4 but should do fine at 1gHz.
I know what Apple says about the minimum RAM for Leopard, but I would be looking for more RAM before getting games. Think a minimum of 1G. I don't have a QS but, for specs I found, it looks like you can install three 512MB sticks for a total of 1.5gHz. THe required PC133 is getting a little pricey but gaming uses a lot of RAM. This is QS RAM from one of my two preferred RAM vendors:
http://www.datamemorysystems.com/apple_info/Apple_PowerMac_Dual_Quick_Silver_1GHz_Memory1246.asp