Look at the back of your case.....does the card block one or two slots at the back of the case. Another way to think of it is if you put 2 screws in to hold it in place, it's a dual slot card, 1 screw = single slot.
The difference is, a single slot card exhausts heat into the case . A dual slot card exhausts hot air outside the case thru an external grille which takes up the extra slot..
Actually it's a bit more complicated. There are cards that wont use two actual slots but the fan/heatsink is large enough that it will cover the slot under it inside your computer, effectively making it a dual slot card. The card Jack linked as single slot is one of them.
I don't believe there are any actual single slot GTS 250s.