Dear Arnoud,
February is considered by most to be the best time to travel in Tanzania. Better later than earlier in the month. Not only are the bulk of the wildebeest migration in the lower Serengeti and Ngorongoro Highlands, but it is calving time and chances are excellent to witness a new wildebeest come into the world.
Naturally there are some bad aspects to this. As it is prime game viewing time, it is also very crowded. Hotels are in peak season prices and if you are the type of person that likes travel in Africa for the remote 'get away from it all' experience, Feb may not be the best month.
The migration remains in Tanzania moving west and then north and then back north east through the end of August. You might also consider June when the migration is in the western Serengeti and prices are lower in most hotels.
I hope this is of help.
Have a great trip,
Gary
http://www.safari.cc For the latest information on East Africa