I had to once, I made the silly mistake of getting two triple slot cards (GTX580). The top card was so starved of airflow. Even after I removed the side panel and took off the GPU shrouds, still overheated in minutes.
Two Arctic Accelero Hybrids later, and GPU temps were as cold as could be. Left the original VRM heatsink on, added memory heatsinks and they worked great for another year or so before I sold them on. One pump did fail on me, but that was probably because of the orientation and the age of them (they weren't exactly a new product) All because I couldn't track down waterblocks for the darn things.
Hybrid coolers work well to keep the GPU cool, it is the memory that I don't like. Without a fan blowing directly on them it seems questionable.