Go to "settings & utilities" in the linode manager. There you can set up alert notifications. It'll email you when you cross a preconfigured threshold with respect to CPU or disk I/O usage. It won't email you about RAM usage (the host machine doesn't know how your Xen VPS is utilizing its RAM); but usually when you max out RAM, you'll hit swap and cause your disk I/O to spike, so you'll get an email anyway when that happens.
