[ubuntu] Dual boot vs. Virtualisation, old question, right?

Have been using dual boot/grub2 (Windows 7 Ultimate x64/Ubuntu 14.04lts 64-bit) for 6-7 months.

Hardware: DIY desktop, AMD FX6300 3.5Ghz, 8GB Ram, one Raptor 150GB(UbuntuOS), one Seagate 1T(WindowsOS).
Everything seems to works fine.

With dual boot, I think I can only use one OS at a time, then shutdown and re-boot each OS to 'switch' to the other OS. (right?)

In visualization, I think I can start on Windows OS (as host), use Virtualbox, for an example, and run Ubuntu as the guest, right?

Whew... QUESTION: When using Windows on the desktop, can I open a window and "peak inside" in the Ubuntu stuff and

share data/use (some)apps for each OS side?

Only one more, please: I see that Ubuntu desktop download can be in "x64" or "x86(for 2GB ram)" - which should I use to

run Ubuntu as a guest?

Thanks, Rick 111101