A link to the tech specs for Photoshop CS6 is provided below if you want to see if/where your system falls short:
http://www.adobe.com/products/photoshop/tech-specs.html
As far as getting a copy of CS5, you can download the trial version of the software using the link below and then use your current serial number to activate it.
Be sure to follow the steps outlined in the Note: Very Important Instructions section on the download pages at this site.
CS5: http://prodesigntools.com/all-adobe-cs5-direct-download-links.html