On our card pages we have a column labeled with "DS" which stands for dual slot. The purpose of this column is to let our readers know if a video card occupies 2 slots. But what does "occupies 2 slots" really mean? I have identified three different kinds of configurations which I think COULD be decribed as dual slot. There are cards that take up two expansion slots on your case that are also 2 slots thick (GTX 295, GTX 285, HD 4870 X2 etc...). These types of cards are dual slot in every sense of the word.
Then there are the two designs shown below. The top is a watercooled HD 4890 from PowerColor. It has a dual height connector plate, but the waterblock on this card is only 1 slot thick. Should this be classified as a dual slot card? I am inclined to say yes. Unless you remove the top part of the connector plate (the vent) you will not be able to install a card directly below it.
The blue card at the bottom is a HD 4670 from HIS. It has a one slot connector plate, a one slot heatsink on the GPU side (not shown) and then a one slot blue heatsink on the top of the card. Technically this card does take up two slots inside the case but normally people are concerned with the thickness of the cooler on the GPU side only. I do not currently label these type of cards as dual slot cards. Do you guys think I should? Let me know in the comments section.
When the new site is ready I think it might be a good idea to change the DS column to allow for numerical values since some cards are 3 slots thick now. 1 would be the default value of course.