There are three different versions of the Dell Optiplex 390: Mini-Tower, Desktop, and Small Form Factor. None have a particularly powerful power supply and would likely require replacement in order to run the 750Ti. While the 750Ti will fit within the PCI-Ex16 slot your GT610 currently resides in, we need to know which 750Ti and which version of the Optiplex 390 you have (provide links, please).
-Wolf sends