Hey Tom in NO,
I believe all North Atlantic traffic from the US and Canada, and from Europe, all have to fly on North Atlantic Tracks during peak traffic hours. This because of the lack of radar coverage over the ocean and the high traffic volumes. North Atlantic Tracks are determined by winds and are plotted twice per day.
Eastbound traffic would select an altitude that maximizes tailwinds, and westbound flights would select an altitude that minimizes headwinds, but they have to fly along a NAT, I believe.
To get an idea of any Great Circle route, check out this website:
However, there is no guarantee that a flight will take a great circle route. Factors like winds, temperature, weather systems, prefered routing, ATC flow management, and ETOPS determine routes that aircraft fly. But generally, the longer the flight, the closer it should resemble a great cirlce route.
I am by no means an expert, so anyone feel free to eloborate or correct me.