There were effectively two "generations" of the G5-the original models that used AGP graphics, and the late '05 models that used PCIe graphics and had a bunch of other changes. The above listing of GPUs is only applicable to the late '05 models.
The AGP computers came with either one or two processors with speed ranging from 1.6ghz to 2.7ghz. As far as I'm aware, anything above 2ghz was dual processor, and anything above 2.5ghz was liquid cooled. If you're buying a dual 2.5 or dual 2.7, look for signs of liquid leaking very carefully, as these are a known trouble spot. The max ram for lower end models was 4gb, while most higher end models maxed at 8gb. These used DDR PC3200 RAM.
The late '05 models used the 970MP processor, a dual CORE version of the G5. These came in a 2.0 dual core, 2.3 dual core, and 2.5 quad core. The last is what's collectively referred to as a "quad." The Quad has two dual core 2.5ghz processors. It is also liquid cooled, but the cooling system is generally pretty reliable.
The late '05 models take DDR2 PC2-4200. They have 8 slots for a maximum of 16gb of RAM.
All G5s have two 3.5" hard drive bays for SATA hard drives. Boot drives must be formatted HFS+/APM, which limits the maximum partition size to 2TB. GUID can be used for non-booting drives, which gives you more flexibility. There can be issues with SATA III drives, but SATA I and SATA 2 are fine. The optical bay has a single slot for an IDE drive, but the IDE bus can support a hard drive if you want to shoehorn one in.
