Yup I switched to Apple Mail from Thunderbird a few months back, primary reason being back then I was barely able to run my developer applications on 512MB of RAM (Apple Mail runs lighter than Thunderbird, unfortunately). It's comforting to find that there is an IMAP-IDLE plugin for Apple Mail (Thunderbird got its IDLE support way back in 2004.)

What's IDLE? Well, quoting myself:

Simply put, IDLE is a command that allows IMAP email servers to transmit updates to the client in real time. This saves the client from having to continuously poll the server to achieve the effect of new mail appearing immediately. All in all, an exceptionally useful command that saves Thunderbird the work of polling IMAP servers continuously.

So anyway, it's really simple to install and use (just simple checkboxes for enabling IDLE for your IMAP accounts).

Preference pane for Apple Mail IMAP-IDLE plugin

Remember to configure Mail to stop polling these mail accounts every X minutes or so now. Enjoy the geeky feeling of knowing you've just saved several CPU cycles and a few kilobytes of bandwidth.