There will be a horizontal gray sticker on the bottom-side of where the side panel lifts off, below the processor cover. That will contain all the specs of what the machine shipped with.
Alternatively, you can tell the model just by looking at the processor cover. If you see two G5 logos on two physically separated half-panels, the machine is a dual processor June 2003 model. If you see one G5 logo on one half-panel, the machine is a single processor June 2003 model. If there are two G5 logos on one aesthetically separated full-panel, the machine is a dual processor June 2004 or April 2005 model. If there is one small G5 logo on one full-panel, the machine is a single processor October 2004 model. If there is one large G5 logo on one full-panel, the machine is a dual core or quad core October 2005 model.
The best GPU not only depends on what expansion slot type the machine is (AGP + PCI or PCIe only), but what OS you intend to run on it. If you want to stay on OS X, the best AGP graphics card available to you would likely be the GeForce 6800 Ultra or Radeon X800 XT, whereas if you plan to use them on Linux or BSD, the best possible card would be the AGP Radeon HD 3850.
Likewise, if the machine(s) are PCIe-based, the best OS X card would be the Quadro FX 4500 or Radeon X1900 GT. On Linux and BSD however, this would instead be any ATI card up to TeraScale 3. A FirePro V5800, for example.
As
@RogerWilco6502 said, all PowerPC G5-based machines support up to OS X 10.5.8. However, there is currently a vibrant community effort underway to get early builds of 10.6 working on PowerPC, which may become a viable option in the near future.
