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.
Apple makes huge margins on their memory prices, for more on that topic, see my Jan 28 entry.
Click the graphs to see current memory prices.
Many users can still do just fine with 12GB. Note that the 6-core model needs the 1333 MHz kit, but it’s only a few dollars more expensive than 1066 so just always get 1333 MHz memory for all models.
Yes, you can put 48GB into a 4/6-core Mac Pro as 3 X 16GB modules. The 8/12-core models can take 48GB as 6 X 8GB, at lower cost.