This paper reports two fundamental observations on the size-dependent optical properties of colloidal gold nanoparticles. First, faceted gold nanocrystals in the size range of 63 +/- 3 nm have been found to be highly efficient for surface-enhanced Raman scattering (SERS). These nanocrystals are identified from a heterogeneous population for large optical enhancement at 647-nm laser excitation. Second, spatially isolated single gold particles emit Stokes-shifted Raman photons in an intermittent on-and-off fashion. In contrast to population-averaged studies, blinking surface-enhanced Raman scattering is a signature of single-particle (or even single-molecule) behavior. By directly measuring optical enhancement and time-resolved emission on single nanoparticles, this work opens new possibilities in studying the mechanisms of SERS, in developing metal tips for near-field optical microscopy, and in designing new nanostructured materials.