Hi,
I have been using Gmail Notifier for a couple of days and although its a nice app I think that it has a memory problem. It is constantly using 35mb of memory and that's, in my world, too much for merely checking to see if there's mail.
Google Talk also has a built in Gmail notification feature and it sits around 10mb memory when you're not chatting with it.
That's a crazy difference since the Google Talk is an IM and not just a notifier.
Just thought I would share.