It all depends on what exactly you mean. Up here its a little different from the states. In the U.S. you have unmaned places where you just throw a quarter into a slot and the thing lets you through. Here, if you mean at the border crossing, its an actual person taking your money. In that case, yes, they do accept USD. I believe its about $2.75 to cross right now.
As for the highways... if you're going to Toronto the only toll highway you may encounter is the 407 ETR. Which you don't deal with anyone at all. When you get on the highway a camera takes a picture of your licence plate and starts to clock how many km's you travel on the highway, then when you exit off it, another camera takes another picture and it calculates what you owe them (so much per km). Then in about a month you'll get a bill. Pretty neat except they have a few little fee's on the bill that they don't seem to mention on thr website. You can calculate your use of the ETR, 407etr.com I believe it is. But the actual bill will be a bit higher due to silly additional fees like camera fees, bla bla bla.
Hope this helped! Have fun!