What do you mean by "simultaneous" users? Do you mean, for example, that an instructor might say to these 800 students, "Begin your exam right now"?
I typically have around 200 students each semester in my online courses, which are asynchronous. With 200 students, seldom are more than 30 "online" at any point in time.
Yes, visit some of the discussion that Visvanath recommends.
I do use GoDaddy myself, and I am not financially invested in GoDaddy nor do I work for them in any way. I am just a GoDaddy customer. I know nothing about Justhost.
The thing to remember is that there are many companies that sell servers, but this doesn't mean that they directly support Moodle. For Moodle support (if you need it), consider a Moodle Partner
. Definitely do not install Moodle with a hosting company's "one-step button."