Thinking of upgrading to vSphere 5.0
Here is quick and handy collection of KBs which you must go through hope you fin dthem useful,
vSphere 5.0 upgrade pre-installation requirements and considerations
http://kb.vmware.com/kb/2004265
Overview of upgrading or migrating from vCenter Server 4.x to vCenter Server 5.0
http://kb.vmware.com/kb/1031307
Upgrading to vCenter Server 5.0 best practices
http://kb.vmware.com/kb/2003866
Migrating to vCenter Server 5.0 using the Data Migration Tool
http://kb.vmware.com/kb/2005328
Issues known to occur after upgrading vSphere 4.1 Update 2 to vSphere 5.0
http://kb.vmware.com/kb/2007404
Methods of upgrading to ESXi 5.0
http://kb.vmware.com/kb/2004501_draft
VMware ESXi/ESX 4.1 and ESXi 5.0 Comparison
http://kb.vmware.com/kb/2005377