Even though sometimes hard drives will function to almost 100% full I strongly disagree with the idea that just 10 GB space might be appropriate.
Allen, I did not say this is optimal - I meant it as the lowest limit, when immediate action is necessary to free more space, before the system will have massive problems. More will certainly be better.
I strongly recommend that all users primarily use a Referenced Originals ("Referenced Masters") work flow with originals/masters stored on external drives separate from the drive that holds the Aperture Library. Those expert both in large databases and in large mass storage solutions may choose to ignore that advice.
For a large Aperture library, referenced is the way to go, no doubt. But as long as the library is still small, only a few projects, and a user has learn how to get the most out of Aperture, I do not see the necessity to introduce the added complication of referenced originals from the start. Once the library grows, it is still possible to take the plunge and go referenced.
Carmitha: Here is a link to the manual page, explaining how to work with referenced original files, so you can make up your mind, if you already want to take this step:
Working with Referenced Images
Regards
Léonie