The only free LEGAL downloads of Mac OS X are the upgrades from 10.0.x to 10.1.5 if you have 10.0.x to 10.1.x purchased version, or 10.2.x to 10.2.8 if you had 10.2 purchased version, or 10.3.. idem, 10.4 idem, 10.5 idem.
Apple sells curretly Mac OS X 10.5 that is too demanding for your hardware. If you have maxxed out the RAM on that hardware, your best bet would be 10.3. For that, the best place would probably be eBay, and the disc will have to be a retail install disc. So black in color, not gray.