We present a catalogue with accurate stellar parameters for the 451 stars that compose the HARPS Guaranteed Time Observations (GTO) ``high precision'' sample. We use our results, together with the others found in the literature, to study the metallicity-planet correlation, namely for very low mass planets. The results presented here suggests that contrarily to their jovian couterparts, neptune-like planets do not form preferentially around metal-rich stars. The ratio of jupiter-to-neptunes is also an increasing function of stellar metallicity. These results are discussed in the context of the core-accretion model for planet formation.