There are a few tools built-in to most Linux distros for gauging and fine-tuning your server's RAM (memory) usage.
The first is a command called free. To use it, simply run:
# free -m
You will see an output like this:
total used free shared buffers cached
Mem: 512 462 49 0 46 127
-/+ buffers/cache: 287 224
Swap: 2047 0 2047
The important figure to look at is the "used" number in the "buffers/cache" row. This will tell you how much memory your processes are currently using. Memory allocation errors will occur if this number is higher than the total amount of memory and swap space. To see how much RAM is free, check the "free" column in the "buffers/cache" row.
Another useful tool is ps, which will show you all of your running processes with RAM usage as a percent of total RAM:
# ps aux
The output will look like this:
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.0 0.1 10368 632 ? Ss Jan07 0:00 init 
root 2 0.0 0.0 0 0 ? S< Jan07 0:00 [migration/0]
root 3 0.0 0.0 0 0 ? SN Jan07 0:00 [ksoftirqd/0]
One of the benefits of a VPS is that, in the case of a memory shortage, you can upgrade RAM on-the-fly with little (Xen) to no (OpenVZ) downtime. Open a support ticket, or find us on live chat, if you need an upgrade!