[dwizzy130]

WM6.5 builds are "classic" WM style. Taskbar(Startbar) on top, softkeys on bottom.
WM6.5.1 and 6.5.3 are all referred to as WM6.5.X or WM6.6. They all have the fat startbar and softkeys on the bottom, and the skinny drop-down taskbar at the top.
Most things for 6.5.X are compatible with most builds of 6.5.X.
The reason the above poster said taskbars may cause your phone to act funny is because to have a custom taskbar requires using some edited dll files that are very important to the system. Sometimes Microsoft will have changed some things inside the dll files that would screw up a build without those changes (or vise-versa).
I hope this helped.
Basically, use caution when installing things like these and take measures such as backing up contacts (and whatever else) before the install.

dwizzy130