At present, Apple sells a quad-core Mac Pro and an 8-core Mac Pro. See my step-by-step guide to how to buy and configure the Mac Pro, and why you would want to buy a refurbished one (especially now, since new models are likely coming soon in 2010). Or you can get a great deal at B&H Photo, here’s the Mac Pro that will give you 90% of the best performance for 50% of the money. Be sure to add memory and drives.
Aside from 4 vs 8 cores, the main limitation of the quad-core Apple Mac Pro has been having four memory slots (vs 8), thus limiting memory to 16GB as four 4GB modules. For most of the model lifespan, Apple didn’t even offer more than 8GB in the quad-core model. Only recently did Apple begin to offer 16GB for the quad-core Mac (at very high cost).
A 16GB memory limit is no practical limit for most Mac Pro users, in fact 12GB is plenty for most Mac Pro users. But some users can use more memory (see Optimizing Photoshop), and in the future, 24GB or more might be the cat’s meow, especially when Adobe finally stops milking its captive franchise, and offers real value with a long overdue 64-bit Photoshop CS5.
Now OWC has the 8GB module solution, thus allowing 32GB in a quad-core Mac Pro.That said, my recommendation would be going to 24GB (3 modules), thus gaining the fastest memory speed. Go to 32GB (4 modules) only if you actually need 32GB. Kudos to OWC for pushing the envelope, and extending the value of the quad-core Mac Pro Nehalem.
The disappointing news? Installing more than 32GB in an 8-core Mac Pro apparently does not work due to a kernel limitation in Mac OS X Snow Leopard. This is quite a surprise (who’s asleep at the wheel at Apple?), but presumably something Apple could fix in 10.6.3. Update: this might not be true after all: 64-bit kernel mode might resolve the issue.