I would say the GTS 250 route since your northbridge does not support DirectX 10.1
What on Earth are you talking about???
The northbridge has nothing to do with it
Because the original design was never intended for it, maybe the it was not released at the time of the MOBOs release, I always felt that updates slow the performance of hardware unless formatted for it.
The motherboard and the current version of DirectX at the time have nothing to do with one another you really need to do some research before handing out inaccurate advice.
Then how come northbridges perform differently in accordance to different video card manufacturers?