already tried with "VMware Hardware Compatibility site" but i couldn't find any help.
Supermicro x8DTU-6+ motherboad - name of the server is SuperServer 1026T-6RF+ (SYS-1026T-6RFT+). When i searched the compatibility page i could find that it is upgradeable up to version 5.1U2 (don't know, if link will work: VMware Compatibility Guide: System Search ), but there is no information about version 5.5
The other server is older and upgrading it is not that important.
we have bought vCenter Server 5 Essentials licence without support. i would try with upgrade by myself, but unfortunately i don't have that chance because of all virtual machines that are running on the server.