Macs tend to "hold their value really well" -- so to be honest, unless you are
specifically yearning for an early 2000s dead-end UNIX workstation that uses a lot of electricity and is extremely good at very specific tasks nobody wants to do anymore (aka the PowerMac G5) then my recommendation is honestly to take a very good and serious look at a brand new Mac Mini:
http://www.apple.com/mac-mini/
I've got the model one generation behind with a Sandy Bridge processor, it plays many modern games on reasonable settings, and in the year or so I've had it, it has a) never been turned off longer than it takes to install more ram B) never made audible noise, save for when I'm really pushing it by playing a game or running some folding.
If you're looking for Mac OS 9 gaming (I wasn't completely sure based on your wording) I would get a fast, single-processor PowerMac G4 and put a
GeFORCE 4 or 4600 TI in it.
Most (all) games on Mac OS 9 will not take advantage of dual processors, (or even that much ram) and to be honest, if you have the physical space available and want to run both Mac OS X and Mac OS 9, I would do it on two physical computers, if for no other reaosn than to make management of the operating system and disks easier. As an incidental benefit, you wouldn't need to turn one off in order to use the other.
On the OS 9 front,
something like this would be perfect, if you could verify that it does not have a Firewire 800 port. (Do not buy an FW800 G4 with intent to convert it, you end up replacing most of the machine and creating more work for yourself in the process.)
Do not bother with the mini or any of the PowerBooks on the OS 9 front, The TiBook@1000 was good in 2003, and stopped being good mid-way through 2005 (and that's being kind to it) and the G4 mini shipped with an extremely similar graphics card. The very fastest eMac fared a bit better with a 128MB Radeon 9600, but it did not run Mac OS 9.
In general though, unless you really do specifically need Mac OS 9, then the mac mini meets your requirements almost exactly, and maybe some 2009 ones with the Penryn Core2 processors and the GeFORCE 9400M IGP are available cheap. (And also $600usd for the current mini is to be honest a fairly good deal if you consider that you're getting Mac OS X, almost enough ram for Mac OS X, and the whole package fits in some cargo pant pockets and uses something like 20 watts at full tilt.)