If you want to update to OS X 10.7 Lion, you'll need to call the online Apple store, which in the US is at 800-MY-APPLE. They'll sell you a download code so you can download it from the App store (which you can now access with your OS X 10.6 upgrade).
To run Lion, the minimum memory is 2GB, however, 4GB is recommended. Click the apple on the top left of the menu bar, then click about this mac to see how much memory you have today.
Also, with Lion, you'll no longer be able to run any PowerPC programs. When people first started installing OS X 10.7 Lion, Office 2004 would no longer work, which took some by surprise (Office 2008 or later is fine).
You can run and download Lion, then install it on an external drive. It will not be speedy, but you will have the chance to see how the applications work or do not work, or to try iCloud, as I understand you have a free iCould account to work with.
Then you can try out without losing your current SL. That would be my plan, at least.