Hi,
Electricity is really expensive: about 1.5$/airconditioner/night, which means that if you run three aircons at night (in 3 different rooms), electricity bills will easily run into 200$ per month. Water is not very expensive (about 40$ per month), unless your house is in Msasani, in which case it has to be brought by truck, and then it's more like 150-200$ per months.
Schools: it really depends where you send your kids. The IST (Int. School of Tanganyika, best school in Dar) costs about 1,000$/month/kid if I'm right.
There are many shcools of course, but educational standards tend to be very low in Tanzania.
There is another fairly good "international school" called the Heaven of Peace, but it's a religious shool and it is located outside of the city center (about 1/2 hour drive, on the way to Bagamoyo).
Leaving on 600$ a month may be possible for a family, but I don't think it's going to be easy.
Hope this helps,
Gilles