Looks like you are looking at the EVGA cards. They are very good, I have the EVGA 780SC, which is a reference 780 with the standard cooler and a factory overclock.
The ACX is a factory overclocked 780 with EVGA's custom cooler, it's supposed to run the card 10c cooler (something along those lines) allowing for higher overclocks if you are so inclined. Although with the ACX design you are dumping hot air back in to the case. With the standard cooler, it dumps it out the rear.
The Classified card is for the extreme overclocker, it has higher default core speed. I think it's also compatible with the EVBot which allows tweaking of a number of aspects on the card; http://www.evga.com/articles/00521/
Depending on how you are set you could by the EVGA reference card and then overclock it to SC speeds through their Precision X software and save a few £/$.