To save a resource email for a task, just archive it by clicking the Archive button above the task. Gmail’s powerful search feature makes it a cinch to find it again later.
With the RTM Add-on for Gmail
If you installed the RTM Add-on for Gmail, life is even better. Use the feature to automatically create tasks when you star or label an email, and RTM links the original email to the task. Read RTM + Gmail = task management goodness and pay special attention to the section entitled Connect Tasks with Your Mail.