Got a G4 loaded with cards, drives, and memory? Don't assume you can move all those goodies to your new G5.
In fact, forget about moving RAM or internal drives. Because the Power Mac G5s are based on the latest memory standards -- 128-bit Double Data Rate Synchronous DRAM (DDR SDRAM), running at 333MHz (PC2700) in the 1.6GHz G5 and 400MHz (PC3200) in the other two models -- no memory from any older Mac will work in the new machines.
(If you're buying third-party RAM for a G5, don't just rely on the specifications -- check with the supplier to make sure the memory has been tested for compatibility with the new architecture. And remember that you can't install DIMMs one by one in the G5, even if they meet all Apple's requirements -- the new machines require matched pairs, just like in the old SIMM era.)
As for internal hard drives, leave them where they are, too. The G5s use a new interface called serial ATA for the built-in hard drive and the one free drive bay. Previous Macs relied on various generations of the older, parallel ATA standard, or in some cases on the completely different SCSI standard, and those drives aren't compatible with serial ATA.
External storage devices -- hard drives, CD burners, Zip drives, and so on -- should work fine, though, via the G5s' FireWire or USB 2.0 ports. (USB 1.1 storage devices, however, will be as slow as ever -- you get USB 2.0's extra speed only when it's built into the peripheral and the host.)
As to existing PCI cards, most but not all will run in the G5s' three free expansion slots. They just have to support "universal," or 3.3V, signaling, rather than 5V signaling. In the 1.6GHz G5 model, the card also has to be capable of running at 33MHz; if you install a card that functions only at 66MHz, "damage to your equipment could result," the G5 manual warns. (Not certain about your cards? Check with the manufacturer.)
The 1.8GHz and dual-2GHz G5s are a bit more forgiving: although the 3.3V requirement still applies, the PCI-X slots in the higher-speed models are backward-compatible with PCI cards running at either 33MHz or 66MHz. That's a good thing, because at press time only one PCI-X card (a Fibre Channel adapter from Astera) was listed in Apple's online Macintosh Product Guide.