Total posts is:
Total threads is:
Then you just have to muck about with the category_list.html template.
Regarding your second question, I have no idea. The best I've been able to do so far is put members/total posts onto my main page, I'd be curious to see how a login would be done. Who's online wouldn't be hard though I don't think.
Here's how I present the Total Threads/Total Posts on my category_list template:
And the simplistic stats on my main site page:
http://www.heavybombers.com (top of page in box)