Remember if you want to go to ESXi 5.1 you need to first be running vCenter 5.1
The upgrade of vCenter 5.1 can be hell depending on what vCenter version you are upgrading from. I know that the latest builds of vCenter 5.1 have addressed most of the hellish upgrade issues people had in the past.
Also be advised that vCenter 5.1 is a complete different animal than 5.0.
Its free to upgrade as long as your support is paid/active.
Yes upgrade 5.1 they just came out with 5.1 Upgrade 1 today so you are falling even more behind.