VBox and Core i7-3770K vs. 3770

Don't worry about a thing.
There is no significant difference between the two aside from the fact that the 3770K is overclockable.

I was in the same situation as you. Even before I bought my i7, I was stuck on the same issue of which version to get: the VT-d or the K. I searched all over, but unfortunately, there didn't seem to be any good answers out there. I would bet that most people never get to buy a 3770 and a 3770K to test out and see which is better.

I would start by asking you what your intentions are with VirtualBox? What do you need to do on these VMs?

I eventually opted for the K because it was on sale really cheap, a bunch cheaper than the 3770. I have been running VBox on a Win 7 host with Win7 and Ubuntu guests. I haven't had any issues with the setup so far other than graphics performance (in games) that is quite a bit slower than running them on the host. I believe it is due mostly to the virtual display adaptor. VT-d is supposed to allow direct access to the graphics card as well as other peripherals, but enabling my BIOS virtualization settings and 3D acceleration in VBox settings I get decent performance after allocating 128MB of video RAM. It allows for Aero to work and my Windows Experience score for graphics is 4.2. It is 7.9 for the host OS.

Unfortunately, I haven't been able to game inside a guest OS on a VT-d system, so I can't tell you how that would work. In any case, if you did want to game, you would probably want to do it on your host OS anyway, because any sort of virtualization (including VT-d) is going to be slower in a guest OS. Give your guest OS a good amount of RAM and it will be just fine with the 3770k.

Good luck!

The principle is simple: If you don't want to heavily overclock your CPU, then the K doesn't offer you any benefit.
Also the 3770 (without K) can be boosted by 4 bins (400MHz) in the turbo modes, so it can clock at a max of 4.3GHz.

The 3770 has the advantage of the VT-d. As mentioned: This can be interesting for you, if you plan to game in a VM. As with VT-d passthrough, you only have a very slight performance loss doing 3D-games in relation to host OS (about 10%). Without VT-d the graphics performance in the VM is much lower.

Like that, you can use a Linux as your Host OS, and a Windows in a VM for gaming.
But as said, this is only interesting if you plan to do HW-Access hungry things in your VM (e.g. can also be used to passthrough a network card etc.)
So if you do stuff in your VM, which uses a lot of HW-Bandwidth you will see a drastic improvement of performance using VT-d.
But be aware, that also your mainboard must support the VT-d, and also the used VM must support it and the corresponding HW.