Short answer: Mac Pro systems have 8 memory slots. I filled them up with cheep low capacity DIMMs. System shipped with 2 1GB sticks, and 2GB sticks fill the rest of the slots.

Long Answer: Bought this Mac Pro in 2008 when they came out, with dual quad core processors. It's been my first desktop since 2002, as I went laptop only for a while. I wanted something crazy fast to see how long I can make it last before feeling the urge to upgrade. With more things going multicore enabled now, the timing seemed right to do it. Sure, I spent a decent chunk of change up front (though not that much more compared to getting some crazy prebuilt gaming box), but overall I think my total cost of ownership of a computer is going to come in lower then cutting corners and having to upgrade sooner. The system is 2.5 years old now, and still exceeding my demands by far. I have to admit, it's really nice to be playing a demanding game, and still have enough idle CPU power left over to transcode videos in the background to sync to my iPad.

As for the RAM, I had initially gone to 6GB. Then a coworker who sold his 2008 quad core Mac Pro to get a 2009 dual quad sold me 8GB of ram for cheep. It's been nice to have, as the OS just caches so much of the files I deal with in RAM, so the hard disks don't get a chance to slow down the computer much. When I'm not using the system, it goes into sleep mode, preserving the cache. Wake on Demand in OS X 10.6 also means the system can be woken by my iPad when I want to transcode a video by simply opening the AirVideo program I use. That works if I'm at home on the local network, or out and about on 3G.