To support vCenter, you only need an x64 version of Windows Server (e.g. 2008 R2, 2012 or 2012 R2 (check which versions are compatible with the vCenter version you want to run). As the 925 supports x64, you can install Windows on it and then install vCenter. I'd suggest at least 4GB RAM, but 6GB gives your server some more breathing space, especially if you plan to also run update manager on this same server.
I was hoping to re-purpose this 860 if it is able to to handle the vcenter install.  The Pentium D pressler 925 is x64 bit?  Any insight would be appreciated!
can vmware vcenter server 5.1 be installed on a physical PE860?