Before you upgrade the ESXi host make sure you upgrade the vCenter to 5.1 and then start upgrading.
Also make sure you do not upgrade the Virtual Hardware on the VM's until all your VM's are upgrade to 5.1
Read http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-51-upgrade-guide.pdf and also check VMware Product Interoperability Matrixes before you plan the upgrade, in addition check if you current hardware is supported for vSphere 5.0