Do you have any particular cooler/card in mind?
Most of the single slot use the ATI reference cooler AFAIK which does indeed tend to run hot, even with the fan set to run faster than stock.
Noise is, at best, subjective: What is a deafening howl to some is a mere rustle to others. Best bet here is to read the Newegg reviews, even if you are not from the US the site is a goldmine of user reviews.
Single and dual slot means the amount of space taken up by the cooler so a 'dual slot' cooler uses one slot but the cooler blocks the one below.
One of the best, in my opinion, is the IceQ series: Cool, quiet and it exhausts its heat out of the case, very handy for hot climates or in a case like mine with less than hurricane force airflow.
Dual slot pro:
-Dual slot provides better cooling
-Dual slot usually expells hot air out of the case (much like I do when I start ranting)
- Render a second slot useless
Otherwise there really isn't a difference. Noise is more a function of cooler and fan quality than single or dual slot.. though usually better cooling performance of a dual slot also means it will be quieter. If you don't need the slot, and have room, there is no reason not to get a dual slot card.