You don't need to worry about that too much. Mac OS X will dynamically assign memory as needed to Logic. Back in the days of OS 9, it was a different story : users needed to allocate memory manually to each application (except for the Finder). Mac OS X dynamic memory management illiminates the need to adjust the memory allocation for each app and it illimates "out of memory" messages.
However, Mac OS 10.4 and Logic 7.2 are 32 bits... so it's impossible for an application to reach more than 4 GB (in practice, it is less). Mac OS 10.5 (Leopard) and the next version of Logic are gonna be 64 bits... this will allow access to MASSIVE amount of memory (we're talking a max of 16 exabytes of RAM!!!)
The moral of the story is that, when you look up memory under Activity Monitor, the more free memory you have(by default the color green) the better. If you have a lot, you're good! 5GB should be more than enough for most project.