1) As was said, Exchange is 1 option.
2) The other choice is the free app named 'Boxcar.' It sends you a push notice instantly when you get G-Mail. So you set Mail to manual IMAP fetch and then let Boxcar send you the push notices instead. When you see that message, you go check mail and it downloads your message at that point.
I use Boxcar instead of Exchange because I like the pop-up message AND I can set it to play a sound from 7 AM to 11 PM and then not play sounds during the night. I really, really enjoy that feature.