Although Apple's requirements for Logic 8 are at least a 1.25GHz or faster PowerPC G4 and 1GB RAM, it still works on slower Macs (500MHz / 512MB). After a quick plist-editing, it will run (or loaf) even on slower G4s with minimal RAM, too. Just open Applications » Logic Pro.app » Control-click and choose Show Package Contents from yhe pop-up menu, then navigate into Contents » Resources and open minsys.plist with your favourite plist editor and change these keys:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070920024543956