Just get a single slot card if your worried about it. A dual slot would probably stay cooler due to the better cooler, but its up to you.
Fastest single slot Nvidia card I saw on Newegg.http://www.newegg.com/Product/Product.aspx?Item=N82E16814187131
GTS450. $145 shipped.
Fastest AMD card I saw on newegg.http://www.newegg.com/Product/Product.aspx?Item=N82E16814131445
6770. $147 shipped.
I'm not sure at work clock speeds the GTS450 needs to be to get close to the 5770/6770. The 6770 is probably the ever so slightly faster card. I don't think its worth the extra price of a normal dual slot card.