I wanted to ask how much RAM is Windows Phone 8 using when we are not using any App etc. Specifically I wanted to ask about Lumia 720, it is a 512 MB RAM device. And how much RAM does Lumia 920 use?
With no apps are running, the Windows Phone OS uses "some" of that memory, and as an end user, you shouldn't need to care, as application developers have to flag their applications as supporting low-memory (256Mb for Windows Phone 7.x; 512Mb for Windows Phone 8) devices as a special case.
Apps that aren't lower memory compatible seem to have a flag in the store that reads "*Not available for all phones" - if you sign in with a Live Id already associated to an incompatible phone, instead of the purchase/install/reinstall button, you'll see a message "This app is not available for your phone.". This seems to be more common for some of the apps that target 7.x, that those that only target windows Phone 8.x