With paintings that old your best bet would be to take them to one of the reputable auction houses, such as Sotherby’s or Christies to have them valued, this will ensure you get an accurate valuation. You will not get this if you just take them to an art dealer as with all antique dealers, they will want to make as much out of them as possible, whereas the auction houses take a set percentage of the sale.
References :
http://www.sothebys.com/
http://www.christies.com/