My tires blew in the freeway while I was going about 75 or so and it was crazy. My steering wheel started shaking really bad and I thought to myself "something's wrong here" and as soon as I thought that, my back tired blew. My car started swerving and I went from the lane next to car pool all the way to the lane next to the exit, did a 360 degree turn around back across all the lanes and ran the back of my car into the center divide. WHEW! Freaky experience....
But yeah, my tires blew because my alignment was bad and the tires I had were really uneven. I was planning on getting them changed but never got around to it....don't make the same mistake...
Anyways, I think you should get all four tires replaced if you change them. Also, get your alignment fixed too so that it doesn't jack up your new tires. It's about 75 bucks or so more but it'll help your new tires last longer. As far as rating goes, I think the the stock rating for your car should be good...but I'm not entirely sure about that so don't quote me on it. Good luck!