245/45/17 is really what you should stick with although you *could* go with a 275/40/17 if you wanted. The sidewall bulge may hurt handling slightly though
A friend of mine actually fit some 315s on his stock Mach1 wheels. It looked pretty ridiculous, but he did it. Personally, I agree not to go any bigger than a 275 on a stock wheel, but 245s look and fit the best.