Whilst the mounting holes on Socket 775 are standard some heatsinks might not fit due to the layout of the components around the socket or if they are tall in a small case. Which heatsink do you have in mind?
Yes, although the mounting mechanism rather than being push-pin looks as though it may need motherboard removal to fit.
Any particular reason for one of these?
I have a spare Intel heatsink for a Quad Core which I'll never use, it's never been fitted and still has the thermal grease on it. If you want it then click on the yellow envelope next to my name and let me have your details. I'll post it later this week if you do.