When should I upgrade the memory on my shared hosting?

Here is the awesome thing:

If you are running your wordpress stock (using the app that webfaction provides) you aren't actually using any of your account memory.

Wordpress is served out of the shared apache instance.

If you are using FastCGI, it will use your accounts memory. There are some benefits to using FastCGI, but if your happy with the performance (and you've already got a caching plugin set-up) I wouldn't change anything.

I upgraded to 512 because I'm going to be running my own instance of Nginx+PHP-FPM+APC. I was running FastCGI (for one site) on the 256 plan and it worked out great.

You can go here to get a bit of code that will add up your memory consumption. I added it to my .bashrc like this:

alias getmem="ps -u <username> -o pid,rss,command | awk '{print $0}{sum+=$2} END {print "'"'"Total"'"'", sum/1024, "'"'"MB"'"'"}'"'

So I can just ssh in and type "getmem" to see my memory consumption. (The goofy "'"'" stuff is to ensure all of the quotes show up right in the command)

You need to ugprade your memory when you have long-running processes for which the total RAM usage exceeds your RAM allowance, or if you are expecting high traffic that will cause those processes to exceed your RAM allowance when it arrives.

If you're running only 5 small Wordpress sites (~40pages each), ~1000 visits/mo each, on our default shared PHP stack, then you probably don't need to worry about upgrading your RAM, ever.