I use MacBiff for that purpose. It just sits in the menubar, and when I've got some mail, beeps and shows the number of new emails. Although you have to enter in your email server information, it doesn't get it from Mail.app, but it does support IMAP and POP, SSL, and multiple accounts.
I just leave it open with the window closed so that all I see is the icon in the dock with a little arrow under it. If new mail comes, I get a pop sound and a highlighted number on the mail icon. It works really well and uses up practically no resources.