If you're willing to pay for power, there's just no beating Directory Opus. http://www.gpsoft.com.au (has screenshots). Of all the GUI file managers, it'd be a fair bet that this is the oldest one still around and in active development. (If I'm wrong please ignore that).
I've been using it since version 4.0, and I've never seen anything more flexible and powerful. Support for FTP, FXP, image viewer and regex searches... plus a ton of other features and even a complete Explorer replacement mode. At least take a look at it.