I have Primegy TX150 S6 and CentOS release 6.9. Commands "uname -a" return "2.6.32-696.el6.x86_64" and "getconf LONG_BIT" return "64", but installed Virtualbox 5.1 not offer 64bit machines!
On Virtualbox's forum I found information that I have to disable Intel Virtualization Technology and VT-d, but cannot find this settings in BIOS.