Hiya,
When you upgrade from Dell's WM2003 to WM5, you need an upgrade disk.
From what I've heard, WM5 on the x50 is not really recommended due the instability.
Take a look at
this thread for starters.
If you really want to upgrade but you do not have the disk, PM
Steven. He will provide you with the disk files. I misplaced mine

.
As for WM6, there is a recent ROM that mag_lite produced. But it is in extreme early stage (I think alpha) and is extremely unstable. It looks like a "suicide ROM"
Here the link (I strongly do NOT recommended flashing it after WM5):
here