Sure, you can replace it with an Ultegra front derailer. Just make sure it's meant for the same size chainrings (I believe most of the Shimano road triple FDs are meant for 52/40/30), and has the appropriate clamp size to fit your seat tube.
But frankly... why?
I ask because a lot of people are unhappy with their front shifting performance and decide that a new FD is what they need. It's usually not gonna get you anything at all... Careful adjustments are almost certain to help you much more. Indexed front shifting is notoriously finicky, and requires careful positioning of the derailer and adjustment of the limit screws to work right. A well-adjusted low-end FD will shift much better than a badly adjusted Ultegra!!!
Here's the Park Tool guide on how to adjust your FD:
http://parktool.com/repair_help/howf...railleur.shtml You'll find no shortage of bikeforums threads on the topic