For the Mac Pro, see Why I Always Have At Least 24GB memory.
Apple memory remains expensive, for more on that topic, see my Jan 28 entry.
Any Mac Pro user with less than 24GB should see whether there is enough memory.
Click the graphs to see current memory prices.
Mac Pro memory prices
Especially if you’re getting a 6-core Mac Pro, the 24GB kit is a no-brainer.
OWC also has a 48GB kit and 96GB kit using 16GB memory modules.
Many users can still do just fine with 12GB, but the 6-core model needs the 1333 MHz kit (only a few dollars more expensive than 1066 so just always get 1333 for all models).