Why does Iran still matter? Aside from Bush elevating them to the status of national bogeyman, they're essentially irrelevant on the national scene.
As long as we are involved in the middle east either in the microscopic level of being in Iraq and Afghanistan, or even at a higher level of the Israel/Palestinian conflict or our Israel policy in general, Iran will matter to some degree.
Now someone could argue the case that we shouldn't be so highly involved in either but that is a much larger debate on America's role as a superpower or as a sort of policemen of the world which neither party takes the anti position on outside of Ron Paul or Dennis Kucinich types. Republicans nor Democrats in the mainstream are isolationists to use that potentially pejorative term.
As far as the actual election I hope Ahmadinejad loses simply because of a lot of his statements on the national stage, not that I suddenly think Mousavi will be some 180 degree flip on most international issues (especially since he won't dictate actual policy on most of these issues because of the structure of their government) but he couldn't be any worse I would think. I'm skeptical Ahmadinejad will lose though.