What's the largest tire that will fit a stock wheel?

After driving in the rain the past few days it's really obvious I need new tires. So I'm looking at some Nitto NT555's for the car. Not the drag radials or road racing tires, but the ones with the 300 treadwear rating. I have a 96 GT with the stock 17 inch split 5 wheels on it. Can I fit the 275's on it? More importantly should I put 275's on it all the way around? Sorry about the semi-newbie question but the local tire stores claim they don't know if this would work.

Thanks for all the help, looks like I'm going with 275s once the I have the money!

Although it is fun being stopped by the ricers and having them ask, "What's your drifting setup?" I told them "Torque." and they didn't get it.