The quickest way I have found to make Command-F default to a name search is as follows...
First, create a smart search (folder), get all the settings the way you want (including the window size, the results bars collapsed or expanded, etc.), put something like Type Filename Here in the search box, and save it as Filename or something. Let it save in your default Saved Searches Folder.
Then, go to that folder, copy the file Filename to the desktop, rename it to default_smart.plist and copy that file to the Resources folder in the Finder app. To get there, navigate to /System -> Library -> Core Services, control-click on Finder, and then navigate to Contents -> Resources. Authenticate when asked, and then use the Activity Monitor or Terminal to restart the Finder. Your new defaults should now be working when you hit Command-F.
[robg adds: This worked as described, though my Finder crashed the first time I hit Command-F ... but I hadn't restarted it myself manually, either (whoops). It's been working fine since then, though.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050621230020563