how to tweak your dock in Leopard
First go to /System/Library/CoreServices and find the "Dock",
right click and press "Show Package Contents".
Then Go inside Contents, then Resources.
Now you're in the right place for making some changes...
You now need to find something to replace your existing dock with.. Leoparddocks is a fine place for just that.
Ok, so now that you have downloaded som nice new png's, you're all set to delete the original ones, and then drag your new dockfiles into place... And the directory you should place your new files was:
/System/Library/CoreServices/Dock.app/Contents/Resources
If you now have replaced the original files with your new ones, go ahead and start the Terminal and type "killall Dock".
Thats it !
ps. if you would like your dock to be 2D, you can achieve just that with the app called Dockscrew (by Superpixel) or with 2DOrNot2D (by Eternal Storms)