Especially if you’re getting a 6-core Mac Pro, the 24GB kit is a no-brainer.
Apple garners fantastic margins on their memory prices, see my Jan 28 entry. So get the minimum from Apple, and save a bundle with OWC memory.
Click the graphs to see current memory prices.
48GB for under $1000
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 or 96GB as 6 X 16GB.
24GB sweet spot for most users
Still a great choice for most users, 24GB (or 32GB if really needed) if very cost effective.
12GB for some users
Many users can do fine with 12GB, though with Lightroom 3 and Photoshop, this can quickly be scarfed up.
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.