I read http://www.anandtech.com/mac/showdoc.aspx?i=2832&p=3. This is a little bit outdate as it is for 2006 Mac Pro.
This is much easy now for 2008’s Mac Pro. The heatsink cover can be easy removed as it is now put in place like Macbook MagSafe Connector. There is no need to pull the memory cage back towards the rear of the case nor pushing the heat sink cover in the opposite direction to unlatch it as explained in the above url
The fan assembly can be removed by just two screws. One attached it to the motherboard at the top right. The other is hidden between plastic fan assembly and the heat sink (bottom right of the fan assembly after removing the heat sink cover). The the picture below showing the hidden screw to loose the fan assembly and on of the magnet to attach the heat sink cover