When Should I Uprgade

I with godaddy shared hosting at the moment and it's going okay, now I have an ecommerce site, blog, and forum, and am wondering when I should upgrade to something better(vps) and if those sites get very busy/active, will it be too hard to move all of it to a vps or dedicated server?

So what I'm getting at is, should I go with a vps now, leave the shared hosting, or should I stay with the shared hosting until sites get busier than move on up?

I'm looking at this vps256 from wiredtree [url]