well you can use an aftermarket heatsink for you cpu (cooler master evo 212 is respected and on the cheaper side). The 290s hopefully you got cards with good cooler designs. I suppose you probably can find just the aftermarket coolers for the cards if you looked, you'd have to swap out the reference cooler yourself then. Beyond that you need to make sure your case has good airflow (generally speaking you intake from the bottom/front and you output from the top/back).
The other alternative would be liquid cooling.