Do expansion cards contribute significantly to overall temp?

The title says it all. If they do, then I was wondering if I should install a case fan to cool em'. If not. then...

If I do get a fan, what king of fan should I get? Im looking for performance, not cool looking leds.

What kind of expansion cards?
Something like a simple modem or PCI controller card, no.
Sound cards can add some heat.
Video cards can go from very little heat, to being the biggest heat generator in the system, by a huge amount. Modern gaming GPU's crank out a ton of heat.
Stay with a good name brand, like the above mentioned Scythe. You should ideally have one 1 case fan blowing in at the front, and one exhausting air out the back, in addition to the PSU fan, which should be exhausting air out the back.
The main thing to check for.... noise, some fans spin too fast, or simply have a poor fin design, and they make a LOT of noise. The better fans are quiet, do not spin so fast, and do a good job of moving air. So unless you want your PC to sound like hovercraft, do some research here.