Can't be able to edit GRUB

Hi!
After a lot of time lost trying to get webpage working (wrong DNS Server) I installed and managed my VMs.
Now I would have a rich and fast graphic on my VMs using my 2 AMD Radeon HD 6850.
I managed to have working the PCI Passtrought but when I try to edit the grub file the pc makes a beep any time that I press a button or it changes line/delete a lot of strings...
How can I use my graphics card correctly?
I hope you'll help me!
Thanks a lot! :D