Apple adds an implementation twist to its memory that might slip by most observers, but which represents a radical change for the better in workstation and power desktop system design. Instead of fixing FBDIMM sockets to the motherboard, as is the norm, Apple placed the memory sockets and supporting circuitry on removable riser cards, minicomputer-style. Like the hard drive trays, the memory risers are held in place with friction and extremely easy to install and remove. There’s no more laying your computer on the floor to mash skinny DIMMs into stiff sockets on a fragile motherboard.
The only disadvantage to easily removable disks, memory and expansion cards is that even though they are not hot-pluggable, meaning they cannot be removed from a powered-up system without the likelihood of damage, they will pull out while the machine is running. The drives have an interlock with the side panel lift lever; you have to raise it to a higher position to pull any of the drives, reducing the likelihood of accidental removal, but you can raise the lever that while the machine is on. There is one of those situations where Apple should relax its policy against LEDs. There's no tip-off from inside the Mac that it’s powered up. There isn’t a single LED on the motherboard, the fans are not readily visible and the machine runs virtually silent. Further, in sleep, the machine seems to be powered down, but pulling memory while it's in this state is just as dangerous as if Mac Pro were in active use. Just get in the habit of pulling the power cord before you open the case.
If you're feeling frisky, get in there with a scope and a soldering iron and poke around for a place from which to tap 5V or 3.3V. It'll be good practice for you if you're the type who plans to follow instructions on the 'net for overclocking this puppy. Apple scowled at me when I asked if Mac Pro's CPUs are field-upgradable. The official word: "I don't think it's possible, but if it is, we won't support that." Given that the CPUs and heat sinks are on risers, too, field performance upgrades and botched attempts at same will be real issues after Mac Pro gets traction in the gaming market, in which area I believe Mac Pro is a strong contender. Apple gets a much bigger installed base, but also a subculture of people who figure out ways to avoid buying Apple's next model. You have to take the bitter with the better, and for my money, it's better to stay wide open. At least sites with the hardware hacks will sell Mac Pros to people who want to try the hacks out themselves.
Apple’s memory and CPU risers make Mac Pro’s cooling much, much simpler and quieter. The memory risers are several inches apart for maximum airflow, and they share the straight line push-pull fan pair that cools the Xeon CPUs. The CPU modules, which are concealed under a plastic panel, are also mounted perpendicular to the motherboard and topped with generous aluminum heat sinks. The heat sinks do not have small, buzzy top-mounted fans.
Air flow is generated by three massive fans, two up front and one in the back. The front and back panels are, top to bottom, a honeycomb in which roughly half of the space is open to air flow. The full-height grilles wrap around a little at the the top allow iMac-style convection cooling when the system is not under load. As a point of interest, the front, top and back of Mac Pro are fashioned from a single piece of aluminum. It doesn't just look slick. It cuts vibration and airflow leakage.