I want to buy http://www.asrock.com/mb/overview.asp?Model=ALiveNF6P-VSTA but it comes with GeForce 6 integrated graphics. It has a PCI-E X16 slot so can I use my existing GeForce 7600 with this mobo? I assume there will be some setting in the bios to disable the on board graphics. I have never had a mobo with on board graphics before so I'm not sure.
You sure as heck can -- most MoBos will default to using PCI-e/AGP slots if components are indeed installed in said slots. If not, as you said there are options in BIOS normal going by the name of "Primary Graphics Display" that usually are set to "Auto" but can be set to PCI, PCI-e, Onboard etc....
Out of curiosity, is there a reason you chose a motherboard with integrated graphics when you don't plan to actually use the onboard video? I've owned 1 ASRock board and only bought it because it was one of the few that supported AGP and PCI-E graphics cards, but in retrospect, I should have just saved more money and bought a better board. They aren't bad boards necessarily, but just curious why you picked this particular one.
From previous experience as soon as you install a new gfx card and drivers it over rules the onboard graphics.