Sofware to monitor system preformances

Our IT department is in the process of restructuring our staff, and were implementing organizationl changes. The group that I’ll belong to is Production Systems which we will focus on the daily operation of generating applications and key business applications and establish a production discipline and consistency in quality control. Our objective is to improve system monitoring & preformances, the problem is that we do not have any of these software tolls in our location. Were looking for software that will help us to improve monitoring and to alert us to problems before they strike. We are an NT and UNIX environment, and cost is a factor to what we what to purcahse. I welcome all comments.