UnDelphiX is simply an updated version of DelphiX. Some individuals took the DelphiX source and made some basic improvements.

I do not know all of them, but the main ones are:

:arrow: Makes use of DirectX 8.1(used to be DirectX 7)
:arrow: Modified to be directly installed into Delphi 7(with the latest version, support for Delphi 6 and below requires some changes)
:arrow: You DO NOT run the .exe in the 'bin' folder you open the .pkg file in the 'source' directory and hit the Install button!

:?: I believe there was some optimizations in the code aswell(mostly using faster methods and avoiding use of slow functions liken to that of cos/sin and accessing memory differently). :?:

That should give you a bit more details. Otherwise same thing really. Pretty much all projects using DelphiX will make use of UnDelphiX as if there was no change(just the improvements, and maybe a removed field in the TDXDraw class constraints I believe).