Nowadays many heat sinks are using adapters (brackets of different sizes) to make fit from one type of socket to another - just to make sure, is your zalman by any chance one of these? (Maybe you had some brackets or such leftover on your first install?)
btw? what cpu are you using?
And what type of heatsink are you looking for?
Check this site out, they have a large list of heatsinks that can help you pick the right one =)Clicky hurr!~