adding image size

I have a job coming up and my client needs a high res file to print 11x17 for a calendar.
No problem. At least thats what i tell them. They dont know that my camera will only capture a file that is 15x10 at 200dpi.
I tried some tests to see how best to add two inches to the height while keeping the dpi at 200.
I tried 4 different methods and its quite hard to tell the difference in a print between them.
What i kept similar in all of them was the way i scaled up two inches. I created an action and went to image - image size.
I went up in half inch increments and changed the mode at the bottom to bicubic smoother each time until i got to 17 inches. This means that yes i went into image size four times and hit ok after each half inch addition. Where i modified the other steps was in the sharpening. I tried sharpening first then upsizing, tried upsizing then sharpening, and lastly tried opening my raw file as a 16 bit prophoto file, upsizing, sharpening, converting to adobe rgb98, mode to 8 bit. For all the extra effort i cant really tell a difference. The file that looked the worst however was the one i sharpened first then upsized.
I now have the word out to sell my d70 camera and am ordering a d200 very soon. Dont tell my wife.