First you should know what features needs, regarding win2012 datacenter and Std it is same option just the HW support is different
Second check the HW specs if it compatible with OS choose
Third you should take in your considerations what win2008 pack installed because some of them not have the ability to upgrade to2012 i.e
"Windows2008 Standard/Enterprise" can be upgraded to win2012 data center and std
"Windows2008 R2 Datacenter with Sp1" can be upgraded to win2012 datacenter but NOT std
That’s some of points that I remember
