When I was getting married (a little over 10 years ago), one of my friends wanted to come to the wedding. It was only after he bought a guidebook that he realized that Tamil Nadu was a long way away from the Taj Mahal and the Himalayas.
"You can not realistically visit both South and North India in a couple of weeks.", I suggested, "just keep it to South India for now and then you can go back and visit Rajastan and Delhi another time."
"Who goes to India and doesn't see the Taj Mahal?" he griped.
It's a common attitude, unfortunately. The New York Times this weekend carried a travel story by a professional travel writer who didn't get to Tamil Nadu until his umpteenth time to India.
Don't get me wrong -- the Taj Mahal is an amazing experience -- but so is the gopuram of the Big Temple in Thanjavur. Even run-of-the-mill temples in South Indian villages are glorious. If you have the chance to go either to North India or to the South, take it. And no, you can't combine it all in one trip -- India is too big for that.
P.S. The Times travel story is good, but the writer commits an unforgivable mistake -- he refers to the cuisine of Tamil Nadu as "Chettinad". I am from Chettinad, and damn if the rest of the state can take credit for our food! It's as if someone were to say "The cuisine of Italy is called Sicilian and it uses a lot of fresh herbs".