I had a holiday there last year, and used Mauly tours, based in Moshi. They organise a car and driver, plus all hotels, and make up an itinnery(?) suitable for your budget.
Its worth stopping off on your way to Serengeti at Ngorogoro for a day or so to split up the journey a bit. There is a hotel on top of the crater with some mind blowing views.
One thing you must do it the balloon safari in the serengeti, it was a fantastic experience. All the hotels in serengeti advertise it.
If you are in a rush for time, catch a charter from Kili straight into the serengeti. These guys are well advertised, but you will miss out on a great drive from Arusha to serengeti. (takes 2 days).
Have fun.