A lot of misunderstandings in this thread:
1) On WP, the amount of RAM has NO influence on how efficiently apps are executed.
2) The amount of RAM has NO influence on how many apps you can install on the device.
3) WP performance doesn't noticeably degrade based on the number of installed apps. Even if it would, RAM wouldn't be important in this regard.
4) When you launch an app on WP8, it is guaranteed access to at least 150 MB of RAM. If that isn't available, tombstoned apps will be closed to free up space.
5) Apps requiring more than 150 MB can't be installed on devices with less than 1GB RAM. So, having only 512 MB of RAM (or less) will restrict your choice of apps, although this will affect only some games (of the "hardcore" variety).
AngryNil mentioned the other scenarios in which less RAM will have an affect.
I'm unaware (and highly doubtful) of a WP8 device coming out with less than 512 MB RAM. If such a device exists, I would avoid it. The supported number of Background Tasks and Multitasking will be too limited.