Parallels migration question current os10.5.8, windows 7 and parallels desktop 4.0

Obviously the OS and the parallels desktop are old.
Daughter has new mac for college so I now have the old mac PowerBook which is configured
OS 10.5.8, Parallels desktop 4.0 and running windows 7N
Assuming there is benefit to upgrade the version of parallels to a newer version - if just for the support aspect.
Not planning to upgrade the OS X or windows at this time.
So with that said what is the best version or potentially the only version of Parallels to move to?

 

to run PD8 you need Mac OS 10.6.8 at least.

But you cannot but PD7 from Parallels website anymore.