The builds I know of are:
- 535u Impulse and Stardock Central (release)
- 537u Stardock Central (beta)
- 538u Impulse (not available anymore because of some problems)
The last build still available is 537u, but you cannot get it from Impulse, you have to install Stardock Central and check "Show pre-release versions" to be able to install it.
See the end of this thread for more information if you want to try updating through Stardock Central.
That beta is still two years old, so don't expect a lot of changes from 535u. If you do not have any problems with the version you are running, my advice will be not to try to update (and if you do, make sure to do backups).
From what I could find, 537u brings the following changes:
- Use of the new weather feed. More reliable, always up to date and now with super-robust location lookup. *Any* location that shows up with accuweather now works with ObjectDock

- x64 version of Windows now can make use of the System Tray and Launch Start Menu functionalities of ObjectDock
Note that the weather feed fix is available without updating ObjectDock, by replacing the stardockweather.ocs file found in C:\Program Files\Stardock\ObjectDock by this version: http://sd.stardock.com/ken/stardockweather.ocx (see this thread for details)