After upgrading to VMware 5.5.0 and you are also considering upgrade the Virtual machine compatibility from v9 to v10 I do recommend performing a snapshot or backup of the VM before hand:

You are no longer able to edit your Virtual machines through the vSphere client in v10!

If you still have users/employees connecting through the regular vSphere client, you might want to consider this or have them access the web client going forward.

Here are the hardware version features to consider:
http://pubs.vmware.com/vsphere-55/index.jsp?topic=%2Fcom.vmware.vsphere.vm_admin.doc%2FGUID-789C3913-1053-4850-A0F0-E29C3D32B6DA.html

http://www.vmware.com/files/pdf/vsphere/VMware-vSphere-Platform-Whats-New.pdf

Links:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2061336