I spent two months in and around northern India. Delhi is one hell of a place – seriosuly intense and I found it tiring. The trains are brilliant though and dirt cheap.
We did a sleeper train to Mumbai from Delhi – in '3rd class' as there was nothing else available. It was a long journey but really fun none the less. Definitely recommend it, although 2nd class (2ac) is probs better as you get less crowds and air con I think. That certainly m akes a difference when its 40 degrees and youre trying to sleep. If you didnt have time for trains, Spice Jet (or spice air) are a budget plane company who will take you from delhi to mumbai for next to nowt.
Watching the programmes on channel 4 at the moment as part of the indian summer season makes me want to go back again.
Red Fort in delhi is def worth a visit. Id do a day trip to the Taj if possible as Agra is pretty much nothing apart from the Taj. If you end up staying up north, go to Pushkar – a little place which is a mecca for pilgrims. Brilliant place and I can recommend accomodation if needed.
Remember to take immodium – you will likely get some form of stomach complaint. I lost over a stone whilst trekking in nepal and living in india – I only weigh 11stone at best of times!
Im seriously jealous. Any questions, let me know!