having several sections to a site

hello,

I am workin on a project...im looking to include a blog, forums, web analytics platform for users and a few other functions as time goes on...should i make it all part of the same site, but users need to keep making user names for the apps?

i dont know what to do. should i make where, you register for one u register for all?

this is a very important part could mean success or death of the site.


any oppinions? Thanks.