Don't know about how much other countries will suffer but if India and Pakistan (hope they never go) go nuclear then it will be Pakistan who will suffer more... why? Simply because India is a huge country type of small continent in itself.... no wonder its called an intercontinental country....
Now look at the natural greenery of both countries.... India is greener and hence source of more healthy oxygen.... look at the size of our vast jungles.... Jim corbet, Sundarbans,Tadoba, Sahyadri western ghats, Eastern ghats, Van Vihar Madhya Pradesh, Gir forest Gujarat, Karnataka forests, Dandeli forest.... heck even Mumbai which is an island city have huge forest with wild cats... India is really wild... my point is all this natural wealth will help reduce or absorb nuclear pollution in India....
Look at Pakistan.... comparatively small country with no jungles, no wildlife, no big cats, no elephants... at most you can find bears, deers, peacocks in very small isolated green areas which they call jungles....
Pakistan will never dare to attack with nuclear bombs on densely populated areas like Delhi, Mumbai Kolkata.... if they do then whole world including China will punish them....