From the specifications you have provided, you can indeed upgrade to Mac OS X 10.5 leopard.
You will need to buy the DVD disk, which can be brought through Amazon or eBay and any other online retail that may stock old version of Mac OS X.
Once you have a copy of the retail DVD disk, insert into the drive and hold down the 'C' while booting.
Follow the onscreen instructions to install.
Best of luck
You'd best shop around for 10.5 as it can get quite expensive depending on the source.