Unable to setup PCI passthrough for display adapter in VirtualBox

VirtualBox supports PCI Passthrough and I'm trying to connect a graphics card to the guest OS (host is Windows Server 2012 and guest is Windows 8.1 Professional, so they should both use the same drivers). I've followed the directions at http://howtoware.blogspot.com/2012/01/oracle-vm-virtualbox-how-to-use-pci.html to set it up (same as the directions in the VirtualBox manual) to get it up and running. The problem is with this step:

VBoxManage Modifyvm "VM Name" --pciattach 02:00.0@01:05.0

This command connects the devices of [PCI bus 2, device 0, function 0] on the host to [PCI bus 1, device 5, function 0] in the guest system. Which destination (guest) address (PCI bus, device, function) should I use to connect the display adapter from the host to the guest? I've tried a lot of different addresses, but VirtualBox reports the following error:

1. If I append to a device address that doesn't exist on the guest, I get this error:

   Failed to open a session for the virtual machine WIN8A.
   
   A virtual device is configured in the VM settings but the device implementation is missing.
   
   A possible reason for this error is a missing extension pack. Note that as of VirtualBox 4.0, certain features (for example USB 2.0 support and remote desktop) are only available from an 'extension pack' which must be downloaded and installed separately (VERR_PDM_DEVICE_NOT_FOUND).
   
   Result Code: E_FAIL (0x80004005)
   Component: Console
   Interface: IConsole {db7ab4ca-2a3f-4183-9243-c1208da92392}
   

   I've made sure the VirtualBox extension is installed for my version of VirtualBox (4.2.xxx).

2. If I append it to a device address that does exist on the guest, I get this error (looks like someone forgot to read the comments in the error definition >_>):

   Failed to open a session for the virtual machine WIN8A.
   
   General failure - DON'T USE THIS!!!. (VERR_GENERAL_FAILURE).
   
   Result Code: E_FAIL (0x80004005)
   Component: Console
   Interface: IConsole {db7ab4ca-2a3f-4183-9243-c1208da92392}
   

Both of these actions power off the VM, I've tried doing this with a brand new VM with Windows 8 (even with a brand new computer the VM won't start).

I have searched for this exception without any luck. Maybe some VirtualBox experts have some answers out there? thanks.