I'm trying to decide which Mac Pro to buy using my ADC discount and if the 8 core is really worth the extra $700 and if I could upgrade to a 8 core later if I went with the quad core. My apologies to you MAC experts out there, if I have committed moral sin by mentioning such a task. As a PC migrating (however slow) to macs, PC users can upgrade CPUs with ease. If the motherboard matches the CPU socket it can work. How upgradable are these Mac Pros?