Ask HN: How do I know if its time to change the host?

I've been running a service on shared host for a few months. The service is getting popular and have started to consume significant proportion of assigned resources (disk space, bandwidth, etc)

Lately, I see my website going down more often than earlier - tends to happen around the time I have more users online or right after a good promotion activity.

How do I know if its time to change the host? Is there anything I could do to find out the number of simultaneous users I can serve on the existing system?

------------------- This is the response from my host:

we have checked and found that at a time 200-300 mysql queries are running through your site so thats why you are getting this issue so i suggest you please take VPS server from us because some time your site taking more CPU and memory usage of the server....