Nehalem turbo boost
I believe the rest of the 5500 series (2.66, 5550 & 2.93 5570) used in the dual processor configurations have higher turbo multipliers, 2/2/3/3. So:
Dual 2.66, 4 or 3 cores active = 2.93 Ghz
" " , 2 or 1 cores active = 3.06 Ghz
Dual 2.93 4 or 3 cores = 3.2 Ghz
" " 2 or 1 cores = 3.33 Ghz
http://www.anandtech.com/showdoc.aspx?i=3597&cp=2
http://www.anandtech.com/showdoc.aspx?i=3597&p=7
One thing I've been wondering about turbo in the new i7 iMac, will they reach these higher clock speeds in the real world? The Pro has an over-engineered cooling system than can handle the 95W TDP with room to spare, but the iMac heatsinks are necessarily less beefy and more confined. How might this affect the ability of the system to reach the max turbo multipliers? It will be interesting to see benchmarks once these machines can be tested.
Edit:
Taking a closer look at that chart, it appear that there are 3 different turbo configs in the Mac Pro models:
single CPU are all 1/1/1/2
dual 2.26 are 1/1/2/2
dual 2.66 and 2.93 are 2/2/3/3