First off, Iran doesn't even have the capacity to enrich uranium to a level that is useable for nuclear weapons. They are being quite forthcoming about these developments for nuclear reactors. But that's not even the point I was going to make...
This same retarded logic was used on Sadam... 'he has nukes so we have to invade because he's a crazy dictator with nukes'. (To paraphrase, don't make me hunt down the exact quotes, thanks)
So, OUR BEST SOLUTION to stopping a crazed dictator with nuclear weapons is to invade his country and give him all the reason to MAKE USE of those same weapons that you are trying to prevent from being used. The thing about a crazed dictator is that he will have a good hiding spot, SO, he'll have ample opportunity as his last expression of power launch those nuclear weapons, and you'll have made matters worse.
That's like trying to stop drunk driving by opening a drive-thru shooter bar.