1.Spammed the F keys on boot, no BIOS
Most commonly it's either Delete or F2.
It always tells you "Press <key> to enter Setup." while counting your RAM and checking for drives, just press that.
1.2 I already know how to get the device manager, how do i disable on board video from there though?
If you're in the device manager window, expand the 'video adapters' item. If you have more than one video card listed there, disable the one that's not the NVIDIA 8400 GS.
To disable, there should be either an option for that directly in the right-click menu; and if not there, select 'Properties' and 'Disable' or 'Turn off' or whatever the button is there.