When you search in OS X 10.4 via Spotlight (Command-Space, or by pressing the Spotlight icon in the menubar), the results appear in a drop-down list, sorted into groups such as Applications, Documents, Folders, etc. Once you have the results visible, you probably know you can select any item in the list with the mouse, or by using the up and down arrow keys. But here are some additional keyboard shortcuts to make selecting your desired match even quicker.
First, if you want to use the Top Hit, just hold down Command and press Return—as soon as you press Command, you’ll see that the Top Hit highlights.
To quickly jump between categories, use Command-Down Arrow and Command-Up Arrow. Once you’re in the desired category, you can then scroll one by one using just the arrow keys. You can jump to the bottom or the top of the entire list with Control-Down Arrow and Control-Up Arrow (or by using the End and Home keys).
And finally, if you want to modify your search term, you can use Control (or Option or Command, it turns out) and Left or Right arrow to jump to the beginning or end of the search field.