澳土及other .NET junkies, 请教string search

If you have used the AJAX Toolkit AutoCompleteExtender, you know it queries a web service continueously as user is typing. Same as when you search on Google or Yahoo, it suggests kewords as you type. My question has nothing to do with, just give you a context.

How to make the server end string as fast as possible. Say I have 50,000 keywords that can be read from a static file into a generic List<> or any other collection type easily. What's the best way to implement the partial string match? say I want to start matching when user types in 2 characters. Doing List.Find() is too slow. Implementing a tree?

澳土, hope all the reading you do can help. Thanks dude.