As for the onboard video, device manager should do the trick but some motherboards require you to disable it in the bios as well(my first one did).

And as for mixing DDR's, theres no way you could. The motherboard is set to a certain spec. Say you have a DDR2 specd MoBo and You put in DDR3, it wont work. Same thing with DDR1.

And as for the video card, it doesnt matter as the ram is built into the video card and it wont be dealing with the mobos spec at all. Only the software will, and the software, unless built to, wont be able to discern either. So there really wont be any problems.