Need HELP choosing Database

Help!! I'm researching relational database systems to convert to. We presently are on a mainframe using key file structures. I want to move off of the mainframe and onto a PC based network (looking at the IBM RSC/6000 as a server) but cannot decide on what database to convert to. We presently write all our own aplications in COBOL and looking to use Visual Basic and/or Visual C++ for the applications on the new platform. I appreciate any insight on the database to use, on the IBM RSC/6000 system I am looking to use, and/or the programming compilers I plan on switching to. OH....some information on our company and our apllications. We are a national manufacturing company with four sites. Main applications are Direct Order Entry, Inventory Control/Tracking, Accounting (AR, AP, GL), and Billing. There are approximately 50-75 users with 3 remote sites on lease lines connected to system. I am on a very tight budget and need to keep cost down. Thanks in advance......JMV