Metallic nanoparticles (NPs) decorated semiconductor nanowires (NWs) heterostructures show significant promise in enhanced optical and opto-electrical properties due to the coupling of surface plasmon to nanowires. Here, we demonstrate a galvanic displacement based strategy to achieve in situ nucleation of Au nanoparticles and then postgrowth into higher order Au nanostructures such as dimers, nanorods, and nanoprisms along the same Si nanowires (SiNWs). The presence of Au nanostructures significantly enhances the optical properties of nanowires. Particularly, a 24 times enhancement of Si Raman scattering signal was achieved with a Au dimer decoration. A Au nanorod aligned in parallel along nanowire strongly enhances the anisotropy of Si Raman scattering, with more than 28 times stronger signal under parallel polarization than that under perpendicular polarization, demonstrating for the first time the surface plasmon enhanced antenna effect. In addition, we demonstrate that surface plasmon enhances photocurrent of SiNW by almost 100%, which is higher than previous reports. Our studies show that SiNWs decorated with metallic nanostructures by in situ galvanic displacement exhibit significant promise toward high efficiency photodetection and light harvesting applications.