I'd say consider upgrading to Snow Leopard, or maybe even Lion. Factors as to which will depend upon yoru specific machine configuration and also what software you currently use. If you have a lot of old software, specifically PPC software, it may have problems under SL and PPC software will not work at all under Lion.
Get more information about your computer. Go to the Apple in the upper left corner of any window, then "About This Mac", then "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).
Snow Leopard 10.6 Technical Requirements - http://support.apple.com/kb/SP575
Macs and Software that will run with Snow Leopard (Mac OS X 10.6.x) - https://discussions.apple.com/docs/DOC-2455
A Mac OS X 10.6 Snow Leopard Application Compatibility List - http://snowleopard.wikidot.com/
OS X Lion Technical Specifications - http://www.apple.com/macosx/specs.html
Installing Lion: Our complete guide - http://www.macworld.com/article/161064/2011/07/installing_lion_complete_guide.ht ml
How to install Lion over Leopard - http://www.macworld.com/article/161087/2011/07/install_lion_over_leopard.html
OS X Lion: Installation Guide, Boot Discs and Q&A - http://www.macstories.net/mac/os-x-lion-installation-guide-boot-discs-and-qa/
Going to SL or Lion will be much cheaper and easier than trying to get hold of Leopard (10.5). Your computer does not limit you to Leopard since you have an Intel Mac.