Using a series of highly regular nanostructures consisting of periodic Ag nanowires fabricated in porous aluminum oxide, we validate the overwhelmingly plasmonic origin of the most intense SERS signals such as those responsible for single-molecule SERS, demonstrating its sensitive dependence on the system's nanogeometry. By varying the interwire gap distance from 35 to 10 nm, the SERS intensity excited with 785 nm laser light, increased over 200-fold. These observations were shown to agree quantitatively with electromagnetic field calculations carried out using the free space green's tensor method.