You can choose to set Agile as your default homepage in JIRA — so you will go straight to GreenHopper whenever you log into JIRA or click the JIRA logo at the top left of any page.
To make it so, go to your user dropdown and select My JIRA Home > Agile:
...note that if you are using Atlassian OnDemand, the dropdown will look like this:
This will result in exactly the same effect as if you had clicked the 'Agile' menu immediately after logging in (i.e. it will automatically take you to your most recently used board in GreenHopper).