Hi, Im planning to migrate vCenter 4.0 U2 on Server 2003 R2 (32-bit) to vCenter 5.0 U1 on Server 2008 R2,
with the aim to then do an inplace upgrade to vCenter 5.1
Hmmm.. vCenter 5.0 is supported on x64 systems only. Do make a check on the upgrade checklists to avoid last min suprise 
Regarding the plugins, they will continue to work as long as they are supported in the new product. For example, I am sure that the heatlth services and status service will work in the 5.0.
While I havent tried out vCenter Storage Monitoring, it seems to be done a little tweaking to work in 5.0. more info here