You could use a Zalman ZM-NB47J (the tall blue one). However, you may need to modify the heatsink (cut off a few fins) since its so close to the PCIe16x slot.
Most active HSF solutions will most likely not work with this board due to clearance problems. The Swiftech MCX-159-R and the Thermaltake A1899 are both too tall. You might be able to use the Cool Master SLC-S41-U1 if you have no caps or ICs interferring with its clearance. The Vantec Iceberg Copper would fit, but it also has a tiny, high RPM fan. However, since its copper, you might be able to undervolt the fan by 25% for slightly quieter operation.
Jab-Tech sells several of these solutions, and NewEgg sells several as well. The Jab-Tech link is
HERE.