Who is your current Web Host ?
http://liquidnetltd.net - DC in Sweden/Stockholm

when your members start complaining or you notice it
when you get 500 errors , when theres a spike in traffic to your site.

depends on you really
generally any offshore shared host will give you some problems at some time .. if you have the resources to go for a VPS, then do it .
If u have enough money just go for VPS
When people start complaining about the loading speed of the site.
When you start getting more hits and pageviews then shift your server.
I didn'r receive any complient from members, and i didn't notice any slowing in my site..
it just the number of visits increased, and in my cp i saw that iam using more than the allowed CPU (allowed CPU = 4.5% , while my forum load was 5.4% yesterday!)
it is not the money problem, but it just that vps, i didn't use it before and didn't know anything about it....