The decision to go nuclear is at this point for Iran essentially a political decision – one that has not yet been taken for the obvious reasons that Iran’s leaders fear the consequences of doing so. The technological capacity is essentially there, but it is quite possible Iran will sit on it for a while. Pakistan had the chance to build a nuclear weapon as early as 1984 but only went public with a nuclear test in 1998. So if Iran even were to assemble a nuclear weapon, it would keep it hidden and quiet, hardly the actions of a country that reportedly would want to use it at the earliest.
And even if Iran were to go nuclear officially, it would obviously never use the weapon unilaterally against, say, Israel. That would invite the utter destruction of Iran, which is the opposite goal of acquiring a nuclear weapon in the first place, survival, and secondarily, influence (you can’t have influence if your country is incinerated). Additionally, it is highly unlikely that the Middle East would go nuclear in response to an Iranian bomb. No Arab state has the industrial or technical capacity to build their own weapons. Furthermore, the United States, which wields considerably more influence in Arab world than in Iran, could exert more influence to prevent this and also potentially offer to provide a nuclear shield at some point to these states. Saudi Arabia, at the most, may buy a weapon off the shelf, but the most likely seller, Pakistan, has proved surprisingly resistant in getting involved in the Iran-Saudi Arabia proxy war. Finally, Turkey, the country in the region with the best ability to follow Iran’s lead is unlikely to go through the trouble and stigma in acquiring nuclear weapons, especially since it is protected by NATO.
Therefore, while unlikely, it would not be a big deal if Iran acquired nuclear weapons.