Title Tag length, search engines and the User

There is no hard and fast rule for the length of a title tag in a web page, at least I haven't seen it on W3.org or anywhere in HTML standards.
Below are my personal findings and views about the length of title tag and how long it should be from web usability point.

Lets first talk about the Search Engine Result:

Length of Title tag of any search listing in Google search engine can be upto 70 characters max, while in MSN it could be up to 75 chars and on the other hand Yahoo search engine shows maximum chars from a title tag of a web page and the maximum length in Yahoo search result of title tag could be up to 118 characters.

Now from user's perspective:
When we talk about user and web usability, the most important thing we need to consider is the Monitor resolution, many users still use 800X600 resolution. Well as we know we can write title of any length*, we should consider only the part of title that is visible in full view of a web page (e.g. in IE) on 800X600 resolution monitor.
Well in my finding this gives me approximate value of title tag length as 80 characters.

I would like to hear your opinions and suggestions on this topic.