how do i know my harddrive is running on ACHI mode

Check the command you are entering in the terminal.  The way you have it entered in your description above, it looks like you may have tried to install virtualbox on an operating system that is runing in a virtualbox environment.  This is one reason that I don’t like the use of the -y switch on most commands issued with sudo.  By eliminating that switch, you would have been forced to confirm the installation of the package and its dependencies where you would have been able to see potential problems and stop the changes from being made.  You may have to start over with this one if you don’t get any display.

I suggest not to change it. 

Using the default boot Mac OS X is able to run both 32bit and 64bit applications. You can confirm this by opening Activity Monitor ~ within the “Kind” column it will show you which applications are running in “Intel”=32bit or “Intel 64bit” mode.

However if you force the kernel to run in 64bit mode it will be exclusive. No 32bit kernel extension will be able to load. Chances are some of your application or maybe even hardware won’t work in that mode.

As for the guest system in VirtualBox you are only limited by it’s support. Unless you are going to allocate more than 4GB of RAM to the virtual machine guest it probably doesn’t make a difference whether you are using Ubuntu 32bit or 64bit