You'd only see performance in an SLI setup if the applications you use support such. Only a hand full of games support SLI configurations so you'd be wasting your money unless you happen to play one or two games full time that support SLI. That's what made ATI's Crossfire platform a better solution as it doesn't require as much application support but I wouldn't touch ATI with a ten foot pole these days...
Generally speaking, all you really need is good air circulation in your case. There are some nice PCI slotted fans that work well considering the little effort required and the price.
Generally speaking, all you really need is good air circulation in your case. There are some nice PCI slotted fans that work well considering the little effort required and the price.