Roger,
One of things that they don't mention in the guide books is the chronic shortage of small denomination bank notes in India. For this reason don't be surprised if you are given a handful of sweets instead of small amounts of change in shops!
I visited Delhi, Agra and Jaipur on a cricket tour 4 years ago. You will have a fantastic time, but be prepared for a huge culture shock, especially as you come out of the airport tired after a long flight. Be prepared to literally barge your way through a phalanx of hawkers and con-artists!
When you are in Agra, make sure that you visit the Taj Mahal - it is absolutely magical. Allow at least 4 hours for your visit - we were only there for a couple of hours and it was way too short. Try and go in the late afternoon so you can watch the sun set behind the dome - you will remember the view for the rest of your life!
One other tip - when you are being mobbed by beggars / street hawkers, try not to make eye contact (mirrored sun glasses are great for this) and don't answer them at all. Even saying "no thank you" tells them that they have your attention, and they will not leave you alone. It seems a bit impolite to just blank them completely, but belive me it is the only way to retain your sanity!!
Have a great trip.
Gary