Shopping for hardware specs is important and remember different types of architecture will offer different benefits. Also, deep packet inspection (DPI) offers significantly better protection than stateful packet inspection; when reviewing performance metrics be sure to consider DPI performance. Here is helpful site:
http://www.sonicwall.com/us/en/products/Deep_Packet_Inspection.html

Processors and memory are obviously important. A good way to know how the product will truly perform in your environment is to take into consideration third-party, independent testing metrics as these will be closer to what type of performance you will actually see. Independent tests should outline the exact conditions that were evaluated and provide a methodology that fully discloses all test conditions.