What like in his city? Just using his spider powers the damage would be minimal I think...a task force could be assembled to take him out. 
If he were to use his brains however and created things such as EMP's then he would be a bigger problem

If he was Punisher's or Captain America's villain, then he would be major threat to them. If he was Iron Man's villain, he would be considered weak and pointless. If he just went on a random rampage, he will be put down within minutes.

Remember that time he went nuts and tried to kill MJ?

I think the idea of Spider-Man being a villian was touched on slightly in the Grim Hunt. Julia Carpenter, being the new Madame Web, revealed to Spider-Man the future if he went through with killing Kraven. She said he would be an equally effective destroyer, and if he didn't kill Kraven, he would save his own life, the thousands of lives upon that, and the millions of lives based on that. I think there was a What if about it too, but I guess according to the Grim Hunt, He would be a supervillian who would kill Thousands, which would cause millions to die as well. In other words, He'll be the most effective mass murderer in the history of the world.

As a stealthy assassin type (with an obvious genius-level intellect) I'd say he could take down quite a few people, but he wouldn't be able to cause mass collateral damage like a Hulk or Thor.

I think the idea of Spider-Man being a villian was touched on slightly in the Grim Hunt. Julia Carpenter, being the new Madame Web, revealed to Spider-Man the future if he went through with killing Kraven. She said he would be an equally effective destroyer, and if he didn't kill Kraven, he would save his own life, the thousands of lives upon that, and the millions of lives based on that. I think there was a What if about it too, but I guess according to the Grim Hunt, He would be a supervillian who would kill Thousands, which would cause millions to die as well. In other words, He'll be the most effective mass murderer in the history of the world.

It's always the nice ones you have to worry about. Spidey's limits have never really been shown. He was killed, then resurrected himself twice as powerful. So yeah, who knows what Pete could do if he was pushed hard enough.

I think Spider-man would cause a lot of damage to people anyway. I don't know if he'll be able to affect the country. But I've read comics where Spidey was bloodlusted and dude kinda scares me.

@Kairan1979: Agreed, I just meant physical collateral damage as the result of a fight.