Thanks guys.
I was thinking of upgrading VCentre to 5.1, then update the hosts. However I've seen KB articles on vmware talking about PSOD after upgrading Vcentre to 5 when hosts are on 4.1, and most of the solutions are to install 4.1 u1.
I think 'd rather update my 4.1 to the current update level, then upgrade vcentre to 5.1, then update hosts again.
for example;