Using Au nanoparticles, it is possible under certain experimental conditions to considerably enhance the sensitivity of a conventional surface plasmon resonance (SPR) device. In this report, we examine the mechanism of this enhancement and discuss the experimental factors that are crucial to the performance of such a nanoparticle based SPR device. Among these factors, the surface plasmon-supporting metal substrate plays a major role. We demonstrate this by comparing experimental SPR data for Au and Ag substrates. In both cases, similar to 25-30 nm diameter Au particles are attached to the substrate metal via a sandwiched monolayer of 1,6-hexanedithiol. The width and the efficiency of SPR for both substrates are affected by the Au particles. However, the particle-induced shift in the SPR angle is observed only for the Au substrate. This observation is explained in terms of competitive effects of propagating surface plasmons in the substrate metal and localized surface plasmons in the Au nanoparticles.