The answer to your core question "How realistically can I expect an employer to visit an online and extended version of my resume?" depends on the type of position and level at which the employee is being hired. In your case, you fall into the student/recent grad/entry level bucket, and in most situations the hiring manager will not take extra time to look for extended information about your work history (or yourself).
This is not to say that you (and students/recent grads/entry level candidates) are not wonderful people with great skills, but rather that there are a lot of you, and in general that extended information elsewhere is not going to dramatically alter my perception of you and where you're coming from. If that information is so incredibly amazing as to tip the scales dramatically in your favor, it'll either be clear to me based on your cover letter that I really should go look because if I don't I'm really missing out, or I'll have heard of you in some other way (personal reference, internet fame, whatever). Not to be harsh, but students/recent grads/entry level candidates are a dime a dozen.
When do I, as an employer, begin to spend more time with a person's resume, and follow links to read extended information and investigate more? When the position is one of greater responsibility, creativity, management of others, and so on -- when I'm evaluating a seasoned professional, for example, or when someone's cover letter is just so amazing that I can't not look. (Not all hiring managers care about cover letters. I am one who does.)