Stored proc's are kept in the SYSTEM tablespace. They're stored in a long column in plain text. You'll need space in the system tablespace for the sum of the char's of your code. But don't cut this too close.

You can "pin" the procedures (compiled code) in memory or choose to have it stored in memory just prior to it's execution. It'll get dumped from memory based on an inventory algorithm that depends on your database version.