Most run 225/40/18. Depending on the width of the rim, you may be able to get away with 215/45/18. try to get as close to OEM diameter as possible so as not to create too much error on your speedometer. Check out the FAQ section, there's a tire size calculator there.
Dont go with 45 series tires!!! I just went to 225/40/18 from my factory 225/50/16 combo. The overall diameter is already over 1 inch bigger (physically measured, not claculated), which is like getting taller rear end gears, not desireable! So combined with the extra weight and taller tires, my car is a pig!