Re: 250W PS with a Radeon x700 Pro
Most newer cards should automatically take over the precedence of the integrated video. In case it doesn't, yes, disable it in the BIOS. Try installing it first to see what happens.
That power supply is ok, if that's what you want. For that price, I would go with something bigger.
*Fact: Microsoft Window's Blue Screen of Death vs Computerforums.org's White Screen of Death. Which is worse?