As long as you either get your speedo/odo recalibrated for the larger tires or compute your MPG's with the corrected miles traveled, then you should not see too much a drop, but you will drop a mpg or two....
The 35's will be heavier then the 33's and they will be heavier then the stock tire so you will have to overcome that. Plus, they are probably wider, so there will be a little more rolling resistence to overcome.
If you keep the stock gears, you might see a bigger drop in city mpg then hwy because you will be 'bogging' the engine more from a stop with the taller tires.
If you only have 3.55 gears, then I would only go to the 33's. You might get away with the 35's with the 3.73 gears, but it'll still 'bog' pretty bad IMO...