Your speedometer/mileage will be about 14% off, i.e., if your speedo says 50, you are actually going about 57mph, and you will have to add 14.29 miles per 100 miles travelded when figuring your mpg. You will also shorten the life of the clutch and you will lose some over all mpg's.

When you do decide to change gears you will want either 4.88's or 5.29's for the 34's. Since a lot of people tend to go bigger and bigger with tires, you should go with the 5.29's when you change the gears. This will give you the option of running 35's or 36's if /when you decide that the 34's just aren't quite big enough.