Yes, just a small amount of traffic and you are forcing people up to VPS, yet VPS default configuration is made to suck the user of money unless they know exactly how to fully optimize the server.
I've been battling with this issue for the last few weeks. I would not say my site's are hugely popular, maybe a couple hundred views per day spread out over 10 domains. They told me I needed to switch to VPS due to server load and I was already looking into it because of how horrible the service was at the time. Starting out with the default configuration my sites were slower than they were with shared hosting (30+ second load times) and ram being used was well over 800MB even though the graph in the panel was displaying it as using less than 400MB total. I trie lowering it to 200mb above the highest spike on the graph and it would go into a reboot loop due to lack of resources. I had to monitor via SSH to see what was really happening.
After a few weeks of trial and error I have finally started to get my sites back in working order.
switched from apache to nginx (i'm hosting wordpress sites)
setup all the needed nginx config files with rewrites and all.
one thing that is needed to look into if you switch to nginx is that they only support PHP 5.2
After I manually changed the server over to PHP 5.3 the ram usage dropped from 800MB to a steady 150MB.
its almost like they market the VPS at people that don't know how to setup servers and just tell them to throw more money there way for need ram, When my exact setup could be used as a default nginx configuration and keep users well below the lowest 300MB of ram option. After all, the configurations I made all came from the dreamhost wiki, why are they not starting people off with a well optimized configuration?