You can have limited iCloud in Snow Leopard. Mail and iCal work fine, but my experience with Address Book didn't go very well.
To upgrade you need:
Check that you computer is compatible with Mountain Lion.
To check the model number hold down the option/alt key, go to the Apple menu and select System information.
- iMac (Mid 2007 or newer) model number 7,1 or higher
- MacBook (Late 2008 Aluminum, or Early 2009 or newer) model number 5,1 or higher
- MacBook Pro (Mid/Late 2007 or newer) model number 4,1 or higher
- MacBook Air (Late 2008 or newer) model number 2,1 or higher
- Mac mini (Early 2009 or newer) model number 3,1 or higher
- Mac Pro (Early 2008 or newer) model number 3,1 or higher
- Xserve (Early 2009) model number 3,1 or higher
Your Mac needs:
- OS X v10.6.8 or OS X Lion already installed
- 2 GB or more of memory
- 8 GB or more of available space
Check to make sure your applications are compatible. Application Compatibility
If you have applications that you want to use that aren't compatible, you can retain Snow Leopard and create a new partition to install Mountain Lion on.
If your computer isn't compatible, you might be able to upgrade to Lion. Computer Compatibility - Lion
If you can/do upgrade, I recommend you make a copy of the installer and move it out of your Applications folder. The installer self-destructs. The copy will keep you from having to downlaod the installer again.