You can go with Business Map Pro or Arcview 8.1 with Business Analyst as an add-on extension. Not knowing your experience in GIS, I would suggest you do exactly what you are doing now asking questions. Second thing to start in on is what is involved with GIS, i.e. hardware, software, metadata, data, the list goes on. The third thing is to understand why you need GIS? Actually, think of it this way, what decision making information is a GIS package going to hopefully supply you with that your current software process can not do.
GIS is a fantastic tool to support the management level decision making. Therefore it is important to know what decisions can support, check out from www.esri.com GIS store the book ArcView Means Business. Right now it may cost cheaper at Amazon.com.
Good Luck and feel free to email if you need any more assistance.
Scott M. Cadigan