Informix vs. Sybase vs. SQL Server vs. Oracle

I am looking for guidance on selecting a database backend which can handle the multiple simultaneous row updates without slowing. Our current call center application is in Visual FoxPro--but the maintenance limitations have become wearing. Our IVR applications are written in VAL and VOS with reporting done in DOS FoxPro 2.6. Microsoft's SQL Server is not looking like a good option anymore, although its licensing and per seat cost are by far the lowest. A contact at another call center tells me that 60 simultaneous row updates on a DEC Proliant with 2 processors and 2 gigs of RAM drop it to a standstill. And we are not using anywhere near that sophisticated equipment. Just your basic high end Pentiums under NT. Oracle caused the company owner to go into sticker shock. Although it appears by far the most stable and is apparently more ODBC compliant than say, Sybase or Informix. I anticipate that Fox is likely to be in place as a front end tool for a while as he began as a Fox programmer and is most comfortable in that environment. Any advice you can give me would be appreciated. Thanks!