With C++ and Visual Studio, making 64-bit builds is generally very easy. You usually don’t have to do much more than recompile with different settings and maybe fix some bugs where pointers are cast to 32-bit numbers and truncated, usually due to Win32 GUI stuff.
That is if there are 64-bit versions, or source code, for all of your (in-process) dependencies. If you can’t make 64-bit versions of the dependencies then things become more complex since 64-bit code cannot load/include 32-bit code in the same process.
I don’t know much about Delphi and 64-bit, though. I’ve read that Borland aren’t going to release an x64 Delphi compiler and apparently want people to switch to Delphi.Net, but that could be old information.
Update: 64-bit support is on the Delphi roadmap for mid 2009, apparently.
For testing, if you don’t have a spare machine to install x64 Windows, you can use VMWare since it lets you run a 64-bit guest OS on a 32-bit host.