I have an older 478 socket pentium lying around with this Zalman cooler attached to it... I was wondering if it is even feasible to mount it on an AM3 motherboard. I am running an Athlon II X3 435 processor in a MSI 785gm-e65 motherboard and would like to improve on the stock cooling.
The short answer is NOT LIKELY, at least without some modification. The HSF you are asking about can be used with an AM2 system, and in most cases you can mount AM2 HSFs to AM3s as long as you use the stock mounting brackets.
In your case, you would need to use the Zalman backplate that is made for an AM2 configuration (2 screws). AM3s use a different mounting to the motherboard itself (4 screws).
Here is a link that shows the parts for your HSF. Looking at the backplate will show you the problem.