This question has arisen several times recently with consulting clients of mine, who were under the impression that a quad-core Mac Pro Nehalem is limited to 8GB memory.
While Apple won’t sell it to you, you can install 12GB (3 modules) or 16GB (4 modules) into a quad-core (single-CPU) Mac Pro Nehalem (see review). Don’t try to mix 4GB modules with smaller ones, it won’t work. An 8-core machine has 8 slots, so you can go to 32GB, an option I don’t recommend (stick to 24GB max eg six modules).
My 8-core dual-cpu 2.93GHz Mac Pro has six 4GB modules (24GB), and my “test mule” 2.66GHz quad-core has three 4GB modules (12GB). I bought the Mule as a refurbished model, saving a cool 15% over new. A Mac Pro is a far better investment than any other Mac.
BTW I have six (6) Apple original 1GB memory modules for sale, contact me if interested.