Next-generation quad-core Intel processors.
The new iMacs use the Intel Sandybridge processors which are 70% faster than before which is really good.
All-new advanced AMD Radeon HD graphics.
The new iMacs have the AMD Radeon HD 6970M graphics processor with 1GB of GDDR5 memory upgradable up to 2GB
Thunderbolt
One of the major changes in the new iMacs is the addition of Thunderbolt ports. These ports allow data transfers up to 10Gbps which is twice as fast as USB 3.0. Great stuff. More and more thunderbolt compatible devices should be out soon. The 21.5 inch version has a single thunderbolt port while the 27-inch version has two thunderbolt ports which enable to connect 2 more monitors to it & have a 3-screen ultimate geek rig! Great stuff!