Should I enable Intel Virtualization and VT-d?

If you use Virtualbox, you should enable both. VT-d does direct I/O. And I believe Intel Virtualization is the same as VT-x.

VT-d allows direct access to the hardware from a virtual machine. Presently VirtualBox does not support this but it won't harm to turn it on. If you install some variety of Linux and use Xen for virtualisation it would support VT-d allowing the direct control of various hardware by a virtual machine such as a 3D graphics card or a sound card.

Virtualbox will require VTx in order to run 64-bit guest OSs. It will not run these if VTx is not available. Anand_trex believes are correct. Having run with these settings enabled on more than one system I don't believe they harm (either performance or stability). It is, however, harmful to run two VTx virtual systems at the same time. Running Virtualbox and, for instance, VMWare at the same time will bluescreen your PC, at least on Windows 7. On Windows 8, I have found that the included Hyper-V outruns all other VMs I have tried.