You should be able to find a retail version of Leopard OS 10.5 on either Amazon or eBay. But you can go directly to OS 10.6 Snow Leopard which is still sold by Apple:
and which would be a much cheaper solution.
Still some help needed.I run 10.4.11 on my
Model Name: iMac
Model Identifier: iMac5,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache (per processor): 4 MB
Memory: 1 GB
Bus Speed: 667 MHz
Boot ROM Version: IM51.0090.B09
SMC Version: 1.8f2
Can I upgrade the OS on this?
Start by checking if you can run Snow Leopard:
Requirements for OS X 10.6 'Snow Leopard'
OS 10.6 Snow Leopard is once again available for $19.99 from the Apple Store:
and in the UK:
but nobody knows for how long it will be available.
To use iCloud you have to upgrade at least to Lion, but some functions are only available in Mountain Lion:
You can also purchase the code to use to download Lion (Lion requires an Intel-based Mac with a Core 2 Duo, i3, i5, i7 or Xeon processor and 2GB of RAM, running the latest version of Snow Leopard), or you can purchase Mountain Lion from the App Store - if you can run that:
When you have installed it, run Software Update to download and install the latest updates for Snow Leopard.