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:
Performance will not be ideal with 3 GB of RAM; although the specs say a minimum of 2 GB, realistically. 4 GB is really a minimum depending on what you use the Mac for. With nothing but a browser open (Firefox, but it's the same with Safari), I am using around 3 GB of RAM. If you do anything with photos, graphics, or movies, then you'd benefit from more RAM. Both Lion and Mountain Lion consume far more - Apple informed me that it was due to the 64 bit addressing rather than the 32 bit (in Snow Leopard and previous).