Right off Apple's website.
Processor and memory
* 21.5-inch and 27-inch models, one of the following:
o 3.06GHz Intel Core 2 Duo processor with 3MB shared L2 cache
o 3.33GHz Intel Core 2 Duo processor with 6MB shared L2 cache
* 27-inch models only, one of the following:
o 2.66GHz quad-core Intel Core i5 processor with 8MB shared L3 cache; Turbo Boost dynamic performance up to 3.2GHz
o 2.8GHz quad-core Intel Core i7 processor with 8MB shared L3 cache; Turbo Boost dynamic performance up to 3.46GHz; Hyper-Threading for up to eight virtual cores
* 4GB (two 2GB SO-DIMMs) of 1066MHz DDR3 SDRAM; four SO-DIMM slots support up to 16GB