it shouldn't overheat any more, did they skim the head? if they didn't you will need another head gasket within a week!!!
If the needle hits the third bar up then you should put heater to full heat and blast, and turn the selector thing to the upright position. the heater in your car is like a radiator, but a bit smaller so it will add to the cooling effect of the coolant system.
oh and don't scrap it, as about 60% of the cars on scrapheaps are junked just because they 'overheated'.
