I agree with Howard. There is no good reason in this day and age to make your site partially SSL. I'd definitely recommend putting the entire site behind SSL.
The only thing you really should be aware of is that you will see warnings for any part of a page which attempts to load content from a non-http source (e.g. you have an image which is loaded from off-site from 
). With things like the Media filters, Moodle does use the HTTPS equivalent where possible (e.g. http://youtube.com becomes https://youtube.com when embedding youtube videos), but not all sites offer HTTPS yet so this is not always possible.
Best wishes,
Andrew