Definitely locked on Zalman's 100% copper coolers. Copper has twice the thermal conductivity of aluminum and this means real tangible results.
I have the mother of crowded cases, with 4 sata drives, 2 video cards, 3 other PCI cards (tuner, WiFi, audio), especially the drives and video cards generating a lot of heat. I'm lucky my case has one of those huge 10-inch side-fans that creates a lot of wind, otherwise I'd be toast.
Practical example: brand-new nVidia fanless card - over 100 degrees Celsius in games. Changed cooler with Zalman VF700 Cu, ~40-45 degrees Celsius in games.