I'll second what Old McLacey says. An Oyster card will save you a lot of money on the tube etc. a travelcard, if you're travelling around more, will save you more and you can get a day or week long one without having to get a photocard etc.
The Travelcard would give you unlimited travel by Train/Bus/Tube within the London Area and I'm pretty sure you could extend it to Hatfield. If this is the better option for you then you don't need to bother with an oystercard, unless you particularly want one!
They do do special "visitor" versions,
http://visitorshop.tfl.gov.uk/, although I would probably wait until you are here to buy one as I don't think it is any cheaper to buy in advance.
Having said that, if you did but it in advance it would get you from Heathrow (within travelcard zone 6) to Hatfield (if extended).
Hope that helps?
